Add C# tool demo

doc/examples/ToolCSharp/README.md

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+Run:
+
+```
+godotenv addons install
+```
+
+Open the project, then restart the editor.

doc/examples/ToolCSharp/SpriteTool.cs

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+using Godot;
+using ImGuiNET;
+using ImGuiGodot;
+
+[Tool]
+public partial class SpriteTool : Sprite2D
+{
+    private bool _showImGui = false;
+
+    public override void _Ready()
+    {
+        if (Engine.IsEditorHint())
+        {
+            _showImGui = ImGuiGD.ToolInit();
+            var io = ImGui.GetIO();
+            io.ConfigFlags |= ImGuiConfigFlags.ViewportsEnable;
+            io.ConfigViewportsNoAutoMerge = true;
+        }
+    }
+
+    public override void _Process(double delta)
+    {
+        if (_showImGui)
+        {
+            ImGui.Begin($"SpriteTool: {Name}");
+            float scale = Scale.X;
+            if (ImGui.DragFloat("scale", ref scale, 0.1f))
+            {
+                Scale = new(scale, scale);
+            }
+            ImGui.End();
+        }
+    }
+}

doc/examples/ToolCSharp/ToolCSharp.csproj

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+<Project Sdk="Godot.NET.Sdk/4.2.2">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <EnableDynamicLoading>true</EnableDynamicLoading>
+    <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="ImGui.NET" Version="1.90.1.1" />
+  </ItemGroup>
+</Project>

doc/examples/ToolCSharp/ToolCSharp.sln

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+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 2012
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ToolCSharp", "ToolCSharp.csproj", "{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+	Debug|Any CPU = Debug|Any CPU
+	ExportDebug|Any CPU = ExportDebug|Any CPU
+	ExportRelease|Any CPU = ExportRelease|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.ExportDebug|Any CPU.ActiveCfg = ExportDebug|Any CPU
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.ExportDebug|Any CPU.Build.0 = ExportDebug|Any CPU
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.ExportRelease|Any CPU.ActiveCfg = ExportRelease|Any CPU
+		{E08468D9-B6AE-4EBA-BE2B-B18B0194FEAF}.ExportRelease|Any CPU.Build.0 = ExportRelease|Any CPU
+	EndGlobalSection
+EndGlobal

doc/examples/ToolCSharp/addons.json

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+{
+    "addons": {
+        "imgui-godot": {
+            "url": "../../../../addons/imgui-godot",
+            "source": "symlink"
+        }
+    }
+}

doc/examples/ToolCSharp/icon.svg.import

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+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://g0surar1un43"
+path="res://.godot/imported/icon.svg-218a8f2b3041327d8a5756f3a245f83b.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://icon.svg"
+dest_files=["res://.godot/imported/icon.svg-218a8f2b3041327d8a5756f3a245f83b.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1
+svg/scale=1.0
+editor/scale_with_editor_scale=false
+editor/convert_colors_with_editor_theme=false

doc/examples/ToolCSharp/main.tscn

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+[gd_scene load_steps=3 format=3 uid="uid://ceybxiw6agvb2"]
+
+[ext_resource type="Texture2D" uid="uid://g0surar1un43" path="res://icon.svg" id="1_vsk0a"]
+[ext_resource type="Script" path="res://SpriteTool.cs" id="2_a7b6t"]
+
+[node name="Node2D" type="Node2D"]
+
+[node name="Icon" type="Sprite2D" parent="."]
+position = Vector2(470, 223)
+texture = ExtResource("1_vsk0a")
+script = ExtResource("2_a7b6t")

doc/examples/ToolCSharp/project.godot

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+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; since the parameters that go here are not all obvious.
+;
+; Format:
+;   [section] ; section goes between []
+;   param=value ; assign values to parameters
+
+config_version=5
+
+[application]
+
+config/name="ToolCSharp"
+run/main_scene="res://main.tscn"
+config/features=PackedStringArray("4.2", "C#", "Forward Plus")
+config/icon="res://icon.svg"
+
+[autoload]
+
+ImGuiRoot="*res://addons/imgui-godot/data/ImGuiRoot.tscn"
+
+[dotnet]
+
+project/assembly_name="ToolCSharp"
+
+[editor_plugins]
+
+enabled=PackedStringArray("res://addons/imgui-godot/plugin.cfg")