StringName Dictionary keys

also added 'is_string()' method to Variant
and refactored many String type comparisons to use it instead

modules/gdscript/tests/scripts/parser/features/dictionary_lua_style.out

@@ -1,2 +1,2 @@
 GDTEST_OK
-{ "a": 1, "b": 2, "with spaces": 3, "2": 4 }
+{ &"a": 1, &"b": 2, &"with spaces": 3, &"2": 4 }

modules/gdscript/tests/scripts/parser/features/dictionary_mixed_syntax.out

@@ -1,2 +1,2 @@
 GDTEST_OK
-{ "hello": { "world": { "is": "beautiful" } } }
+{ "hello": { &"world": { "is": "beautiful" } } }