C#: Add initial implementation of source generator for script members

This replaces the way we invoke methods and set/get properties.
This first iteration rids us of runtime type checking in those
cases, as it's now done at compile time.
Later it will also stop needing the use of reflection. After that,
we will only depend on reflection for generic Godot Array and
Dictionary. We're stuck with reflection in generic collections
for now as C# doesn't support generic/template specialization.

This is only the initial implementation. Further iterations are
coming, specially once we switch to the native extension system
which completely changes the way members are accessed/invoked.
For example, with the native extension system we will likely need
to create `UnmanagedCallersOnly` invoke wrapper methods and return
function pointers to the engine.

Other kind of members, like event signals will be receiving the
same treatment in the future.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.props

@@ -2,6 +2,6 @@
   <ItemGroup>
     <!-- $(GodotProjectDir) is defined by Godot.NET.Sdk -->
     <CompilerVisibleProperty Include="GodotProjectDir" />
-    <CompilerVisibleProperty Include="GodotScriptPathAttributeGenerator" />
+    <CompilerVisibleProperty Include="EnableGodotGenerators" />
   </ItemGroup>
 </Project>