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
We finally have everything in place for distributing the Extensions app
as flatpak without jumping through too many hoops. Add a manifest that
can produce a nightly build like other GNOME modules, and can serve as
a template for a stable manifest on flathub.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
