feat: establish release channels

[onbjerg]

I think we need a better construct for release channels. For nightly the setup in this PR works fine, but for stable it does not: the primary tag of the release is not a stable-$SHA tag, it's a vx.y.z tag. I looked at GitHub's API and there is no way for us to get all tags pointing to a specific commit unless we literally just iterate over all tags in the repository (worst case).

An alternative would be to have some file that just describes the latest release in each channel like:

nightly: nightly-$SHA
stable: v1.0.2

Note that it is not a JSON file or something similar: it is easy to parse in both Node.js (for the action) and Bash (for foundryup). The question is where to host this file.

Originally posted by @onbjerg in #492 (comment)

[lucas-manuel]

Close-able? @mds1

[mds1]

Leaving open for v1.0.0 since we might have a different release process at that point, assigning to @Evalir

[mds1]

Consider release checksums as mentioned in #495 as part of this, along with apt/brew distributions in #20