Change template syntax

Templates now use a TypeName instead of an identifier, which makes it
clearer that it's an extern symbol (or that it's a Gtk.ListItem).

tests/sample_errors/legacy_template.blp

@@ -0,0 +1,10 @@
+using Gtk 4.0;
+
+template TestTemplate : ApplicationWindow {
+  test-property: "Hello, world";
+  test-signal => $on_test_signal();
+}
+
+Dialog {
+  transient-for: TestTemplate;
+}