C#: Enable exporting nodes to the inspector

[raulsntos]

Follow-up to #62185 that implements godotengine/godot-proposals#1048 for C#.

We may want to wait for the dotnet6 branch to be merged first, in which case I can remake this PR.
For the dotnet6 branch, the implementation would have to be moved to:

godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/ScriptPropertiesGenerator.cs

Lines 395 to 404 in 56fd0b8

	if (variantType == VariantType.Object && type is INamedTypeSymbol memberNamedType &&
	memberNamedType.InheritsFrom("GodotSharp", "Godot.Resource"))
	{
	string nativeTypeName = memberNamedType.GetGodotScriptNativeClassName()!;
	hint = PropertyHint.ResourceType;
	hintString = nativeTypeName;
	return true;
	}

[neikeq]

It's fine to merge it before dotnet6. This one will be easy to rewrite while rebasing.

[akien-mga]

Thanks!