Auto build a dev release with Github Actions

[bjesus]

Hi! I'm not sure if this is something you're interested in, but I thought it could help other try out TabFS faster.

This PR doesn't change any TabFS code. Instead it just adds a GH Action workflow that compiles the filesystem (for Linux only at this point, as I couldn't test it on a mac), and packs the extension using Mozilla's Web-ext. It then creates a "pre-release" holding the two assets - a tar file the one can just untar and run "install.sh", and a zip file the one can load onto Firefox Developer Edition.

It automatically runs after every commit.

I would love to explore how I could improve this to help Chrome / Mac users too, in case someone finds this interesting.

[osnr]

Hmm. This is good work, but I'm not sure I actually want to make it easier to install just yet (especially if it's at the cost of making the extension a sealed artifact that people can't modify). Will leave this open and think about it.