Enable LSP go-to-definition, find-all-references, and go-to-implementation with command handler precedence (#80951)

## Changes: Enable LSP Go-to-Definition, Find-All-References, and
Go-to-Implementation Capabilities

Allows other extensions to call into our LSP handlers, but ensures that
the MEF APIs are still used for normal user invoked commands by ensuring
our MEF handlers are ordered before LSP MEF handlers.

### Completed:
- [x] Enable `DefinitionProvider`, `ReferencesProvider`, and
`ImplementationProvider` capabilities in
`AlwaysActivateInProcLanguageClient.GetCapabilities()` regardless of the
LSP feature flag
- [x] Add `Order(Before = ...)` attribute to
`GoToDefinitionCommandHandler` to ensure it runs before any LSP command
handlers
- [x] Add `Order(Before = ...)` attribute to
`FindReferencesCommandHandler` to ensure it runs before any LSP command
handlers
- [x] Add `Order(Before = ...)` attribute to
`GoToImplementationCommandHandler` to ensure it runs before any LSP
command handlers
- [x] Add new constants `LspGoToDefinition`, `LspFindReferences`, and
`LspGoToImplementation` to `PredefinedCommandHandlerNames`
- [x] Update command handler name values to exact requested strings
- [x] Document that LSP command handler names are externally agreed upon
and cannot be changed
- [x] Build verified successfully

### Files Modified:
1.
`src/EditorFeatures/Core/LanguageServer/AlwaysActivateInProcLanguageClient.cs`
- Enabled DefinitionProvider, ReferencesProvider, and
ImplementationProvider capabilities unconditionally
2.
`src/EditorFeatures/Core/GoToDefinition/GoToDefinitionCommandHandler.cs`
- Added Order(Before) attribute for LSP handler
3.
`src/EditorFeatures/Core/GoOrFind/FindReferences/FindReferencesCommandHandler.cs`
- Added Order(Before) attribute for LSP handler
4.
`src/EditorFeatures/Core/GoOrFind/GoToImplementation/GoToImplementationCommandHandler.cs`
- Added Order(Before) attribute for LSP handler
5.
`src/EditorFeatures/Core/Extensibility/Commands/PredefinedCommandHandlerNames.cs`
- Added LSP command handler name constants with exact values and
documented they are externally agreed upon

### Summary:
These changes enable clients to call the LSP go-to-definition,
find-all-references, and go-to-implementation handlers by advertising
the capabilities in the AlwaysActiveInProcLanguageServer. The
traditional command handlers are ordered to run before any LSP-based
command handlers, ensuring they remain active for normal scenarios while
allowing experimentation with LSP handlers.

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src/EditorFeatures/Core/Extensibility/Commands/PredefinedCommandHandlerNames.cs

@@ -191,4 +191,22 @@ internal static class PredefinedCommandHandlerNames
     /// Command handler name for showing the Callstack Explorer tool window.
     /// </summary>
     public const string ShowCallstackExplorer = "Show Callstack Explorer";
+
+    /// <summary>
+    /// Command handler name for LSP Go To Definition.
+    /// This name is agreed upon externally and cannot be changed.
+    /// </summary>
+    public const string LspGoToDefinition = "LSP GoToDefinitionCommandHandler";
+
+    /// <summary>
+    /// Command handler name for LSP Find References.
+    /// This name is agreed upon externally and cannot be changed.
+    /// </summary>
+    public const string LspFindReferences = "LSP FindReferenceCommandHandler";
+
+    /// <summary>
+    /// Command handler name for LSP Go To Implementation.
+    /// This name is agreed upon externally and cannot be changed.
+    /// </summary>
+    public const string LspGoToImplementation = "LSP GoToImplementationCommandHandler";
 }

src/EditorFeatures/Core/GoOrFind/FindReferences/FindReferencesCommandHandler.cs

@@ -15,6 +15,7 @@ namespace Microsoft.CodeAnalysis.FindReferences;
 [Export(typeof(ICommandHandler))]
 [ContentType(ContentTypeNames.RoslynContentType)]
 [Name(PredefinedCommandHandlerNames.FindReferences)]
+[Order(Before = PredefinedCommandHandlerNames.LspFindReferences)]
 [method: ImportingConstructor]
 [method: SuppressMessage("RoslynDiagnosticsReliability", "RS0033:Importing constructor should be [Obsolete]", Justification = "Used in test code: https://github.com/dotnet/roslyn/issues/42814")]
 internal sealed class FindReferencesCommandHandler(FindReferencesNavigationService navigationService)

src/EditorFeatures/Core/GoOrFind/GoToImplementation/GoToImplementationCommandHandler.cs

@@ -16,6 +16,7 @@ namespace Microsoft.CodeAnalysis.GoToImplementation;
 [Export(typeof(ICommandHandler))]
 [ContentType(ContentTypeNames.RoslynContentType)]
 [Name(PredefinedCommandHandlerNames.GoToImplementation)]
+[Order(Before = PredefinedCommandHandlerNames.LspGoToImplementation)]
 [method: ImportingConstructor]
 [method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
 internal sealed class GoToImplementationCommandHandler(GoToImplementationNavigationService navigationService)

src/EditorFeatures/Core/GoToDefinition/GoToDefinitionCommandHandler.cs

@@ -28,6 +28,7 @@ namespace Microsoft.CodeAnalysis.GoToDefinition;
 [Export(typeof(ICommandHandler))]
 [ContentType(ContentTypeNames.RoslynContentType)]
 [Name(PredefinedCommandHandlerNames.GoToDefinition)]
+[Order(Before = PredefinedCommandHandlerNames.LspGoToDefinition)]
 [method: ImportingConstructor]
 [method: SuppressMessage("RoslynDiagnosticsReliability", "RS0033:Importing constructor should be [Obsolete]", Justification = "Used in test code: https://github.com/dotnet/roslyn/issues/42814")]
 internal sealed class GoToDefinitionCommandHandler(

src/EditorFeatures/Core/LanguageServer/AlwaysActivateInProcLanguageClient.cs

@@ -118,6 +118,15 @@ public override ServerCapabilities GetCapabilities(ClientCapabilities clientCapa
 
         serverCapabilities.SpellCheckingProvider = true;
 
+        // Enable go to definition, find all references, and go to implementation capabilities for experimentation.
+        // These are enabled regardless of the LSP feature flag to allow clients to call these handlers.
+        serverCapabilities.DefinitionProvider = true;
+        serverCapabilities.ReferencesProvider = new ReferenceOptions
+        {
+            WorkDoneProgress = true,
+        };
+        serverCapabilities.ImplementationProvider = true;
+
         return serverCapabilities;
     }