extensions-app: Rename executable

We kept the old binary name when overhauling the tool for the Extensions
app to avoid unnecessary churn for packagers/distributors.

However we now have a reason to "free" the old name, so rename the binary
to match the (sub)project name.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1220

subprojects/extensions-app/js/main.js

@@ -39,7 +39,7 @@ function toggleState(action) {
 var Application = GObject.registerClass(
 class Application extends Gtk.Application {
     _init() {
-        GLib.set_prgname('gnome-shell-extension-prefs');
+        GLib.set_prgname('gnome-extensions-app');
         super._init({ application_id: 'org.gnome.Extensions' });
     }