Release master build as separate extension

[mgalgs]

I've been maintaining a fork at mgalgs/gnome-shell-system-monitor-applet with some GitHub Actions hooked up to continuously sync with this repo and release builds off of the tip of master as system-monitor-next on extensions.gnome.org. This is nice for anyone on bleeding edge distros ("I use Arch btw" :stuck_out_tongue_winking_eye: ) who doesn't want to install from source.

This has been working fine but it might be preferable to bring that into this project to make it a little more official.

Should work mostly out-of-the-box after a bit of trimming (won't need the "syncer" action, just the "uploader" one). If the maintainers are interested I can prepare a PR.

[mgalgs]

One drawback of bringing these actions into this repo is that a maintainer would need to add their extensions.gnome.org login credentials as a secret on this repo, which would be readable by any other maintainer (since they could spin up an Action to just print out the secret).