extensions-app: Move Extensions app to new subproject

The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.

This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133

.gitlab-ci/run-eslint.sh

@@ -23,7 +23,7 @@ run_eslint() {
   mkdir -p $(dirname $output)
   touch $output
 
-  eslint -f unix ${!extra_args} -o $output js
+  eslint -f unix ${!extra_args} -o $output js subprojects/extensions-app/js
 }
 
 list_commit_range_additions() {