Add HtmlTextWriter and required support

This is a port of HtmlTextWriter with the following changes:

- `HtmlTextWriter.[Enter|Exit]Style(...)` methods (4 total) are not supported. The `Style` class brings in a lot of additional stuff we probably don't want to support
- HashTable usage has been switched to Dictionary<,>
- Manual array management has been switched to lists
- Helper structs have been made into readonly records to simplify definition and usage
- Annotated for nullability

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/ExcludedAttributes.txt

@@ -1 +1,2 @@
 T:System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute
+T:System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/Header.txt

@@ -17,4 +17,7 @@
 #pragma warning disable CS0809 // Obsolete member overrides non-obsolete member
 #pragma warning disable CA1063 // Implement IDisposable Correctly
 #pragma warning disable CA1816 // Dispose methods should call SuppressFinalize
+#pragma warning disable CA1721 // Property names should not match get methods
+#pragma warning disable CA1711 // Identifiers should not have incorrect suffix
+#pragma warning disable CA1720 // Identifier contains type name
 

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/TypeForwards.Framework.cs

@@ -17,6 +17,9 @@
 #pragma warning disable CS0809 // Obsolete member overrides non-obsolete member
 #pragma warning disable CA1063 // Implement IDisposable Correctly
 #pragma warning disable CA1816 // Dispose methods should call SuppressFinalize
+#pragma warning disable CA1721 // Property names should not match get methods
+#pragma warning disable CA1711 // Identifiers should not have incorrect suffix
+#pragma warning disable CA1720 // Identifier contains type name
 
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.HttpApplication))]
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.HttpApplicationState))]
@@ -69,3 +72,8 @@
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.Hosting.HostingEnvironment))]
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.SessionState.HttpSessionState))]
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.SessionState.SessionStateMode))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.UI.HtmlTextWriter))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.UI.HtmlTextWriterAttribute))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.UI.HtmlTextWriterStyle))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.UI.HtmlTextWriterTag))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.UI.WebControls.HorizontalAlign))]

src/Microsoft.AspNetCore.SystemWebAdapters/HttpResponseWrapper.cs

@@ -4,7 +4,6 @@
 using System.Collections.Specialized;
 using System.IO;
 using System.Text;
-using Microsoft.AspNetCore.Http;
 
 namespace System.Web
 {

src/Microsoft.AspNetCore.SystemWebAdapters/Internal/HttpValueCollection.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections;
 using System.Collections.Specialized;
 using System.Text;
 using System.Web;