Add MustBeVariant attribute and analyzer

- MustBeVariant attribute can be used to enforce that generic types must
be a marshable from/to Variant.
- Also renames all diagnostic ids to be valid unicode identifiers.

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

@@ -49,7 +49,7 @@ namespace Godot.SourceGenerators
 
             if (godotClasses.Length > 0)
             {
-                var typeCache = new MarshalUtils.TypeCache(context);
+                var typeCache = new MarshalUtils.TypeCache(context.Compilation);
 
                 foreach (var godotClass in godotClasses)
                 {