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
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
