Add C# source generator for a new ScriptPath attribute

This source generator adds a newly introduced attribute,
`ScriptPath` to all classes that:

- Are top-level classes (not inner/nested).
- Have the `partial` modifier.
- Inherit `Godot.Object`.
- The class name matches the file name.

A build error is thrown if the generator finds a class that meets these
conditions but is not declared `partial`, unless the class is annotated
with the `DisableGodotGenerators` attribute.

We also generate an `AssemblyHasScripts` assembly attribute which Godot
uses to get all the script classes in the assembly, eliminating the need
for Godot to search them. We can also avoid searching in assemblies that
don't have this attribute. This will be good for performance in the
future once we support multiple assemblies with Godot script classes.

This is an example of what the generated code looks like:

```
using Godot;
namespace Foo {
	[ScriptPathAttribute("res://Player.cs")]
	// Multiple partial declarations are allowed
	[ScriptPathAttribute("res://Foo/Player.cs")]
	partial class Player {}
}

[assembly:AssemblyHasScripts(new System.Type[] { typeof(Foo.Player) })]
```

The new attributes replace script metadata which we were generating by
determining the namespace of script classes with a very simple parser.
This fixes several issues with the old approach related to parser
errors and conditional compilation.
It also makes the task part of the MSBuild project build, rather than
a separate step executed by the Godot editor.

3 files changed, 57 insertions, 0 deletions

diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Bar.cs b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Bar.cs
new file mode 100644
index 0000000000..5eaebc4474
--- /dev/null
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Bar.cs
@@ -0,0 +1,15 @@
+namespace Godot.SourceGenerators.Sample
+{
+    partial class Bar : Godot.Object
+    {
+    }
+
+    // Foo in another file
+    partial class Foo
+    {
+    }
+
+    partial class NotSameNameAsFile : Godot.Object
+    {
+    }
+}
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Foo.cs b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Foo.cs
new file mode 100644
index 0000000000..21a5bfe560
--- /dev/null
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Foo.cs
@@ -0,0 +1,11 @@
+namespace Godot.SourceGenerators.Sample
+{
+    partial class Foo : Godot.Object
+    {
+    }
+
+    // Foo again in the same file
+    partial class Foo
+    {
+    }
+}
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Godot.SourceGenerators.Sample.csproj b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Godot.SourceGenerators.Sample.csproj
new file mode 100644
index 0000000000..24f7909861
--- /dev/null
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Godot.SourceGenerators.Sample.csproj
@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.1</TargetFramework>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <!-- $(GodotProjectDir) would normally be defined by the Godot.NET.Sdk -->
+    <GodotProjectDir>$(MSBuildProjectDirectory)</GodotProjectDir>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <!-- The emitted files are not part of the compilation nor design.
+    They're only for peeking at the generated sources. Sometimes the
+    emitted files get corrupted, but that won't break anything. -->
+    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
+    <CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)\GeneratedFiles</CompilerGeneratedFilesOutputPath>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\glue\GodotSharp\GodotSharp\GodotSharp.csproj">
+      <Private>False</Private>
+    </ProjectReference>
+    <ProjectReference Include="..\Godot.SourceGenerators\Godot.SourceGenerators.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+  </ItemGroup>
+
+  <!-- This file is imported automatically when using PackageReference to
+  reference Godot.SourceGenerators, but not when using ProjectReference -->
+  <Import Project="..\Godot.SourceGenerators\Godot.SourceGenerators.props" />
+
+</Project>