language: Support boxed types and GType

- Add support for type checking boxed types
- Remove support for converting string and number literals
- Add the `typeof()` operator for GType literals

tests/samples/parseable.blp

@@ -1,5 +1,14 @@
 using Gtk 4.0;
+using Gio 2.0;
 
 Gtk.Shortcut {
   trigger: "Escape";
 }
+
+Picture {
+  paintable: "/path/to/paintable";
+}
+
+ColorButton {
+  rgba: "rgb(0, 0, 0)";
+}