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/typeof.blp

@@ -0,0 +1,11 @@
+using Gtk 4.0;
+using GObject 2.0;
+using Gio 2.0;
+
+Gio.ListStore {
+  item-type: typeof(GObject.Object);
+}
+
+Gio.ListStore {
+  item-type: typeof(.MyObject);
+}