GDScript: Don't coerce default values to the export hint type

This behavior is inconsistent with non tools builds and can create
issues. Instead, a warning is emitted if there's a type mismatch. If the
type can't be converted, an error is shown instead.

For the editor it gives a converted value to avoid issues with the
property editor, which expects the correct type.

modules/gdscript/gdscript.h

@@ -304,6 +304,7 @@ struct GDScriptWarning {
 		UNSAFE_CALL_ARGUMENT, // Function call argument is of a supertype of the require argument
 		DEPRECATED_KEYWORD, // The keyword is deprecated and should be replaced
 		STANDALONE_TERNARY, // Return value of ternary expression is discarded
+		EXPORT_HINT_TYPE_MISTMATCH, // The type of the variable's default value doesn't match its export hint
 		WARNING_MAX,
 	} code;
 	Vector<String> symbols;