We currently include a fake config.js file to satisfy the indirect
import from ExtensionUtils. However we're about to need to pass
build-time information into the program ourselves, so generate
a proper file.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2841>
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
