Support Forgejo (& GHES?)

[cswimr]

Currently, this action uses regex to check specifically for github.com. This means that part of this action's functionality cannot be used in its current state on other platforms, or in GitHub's own enterprise selfhosting version. This PR adds support for non-GitHub providers, although I've only specifically tested it with Forgejo.

[cswimr]

I'm not able to build this branch (or master for that matter) on my local development machine. Full disclosure, these changes were made by me a few months ago, I just hadn't contributed them upstream yet and wanted to clean them up a bit before doing so. I know the changes work, but I can't test this PR easily because I'm unable to build the dist files the action actually uses. Here's my old, known-good changes to compare against, as well as an example job run.

[cswimr]

Alternatively to setting the default to github.com, it may be beneficial to set the default to the contents of the GITHUB_SERVER_URL environment variable. Judging by the name, I'm assuming this is a thing in GitHub Actions, and it certainly exists in Forgejo Actions, where it returns the instance URL of the Forgejo instance that the action is running against. This creates an edge case where the action could potentially break if the Forgejo instance's SSH_DOMAIN is set to something other than the ROOT_URL, but I think that's probably a relatively rare issue and it's easy to work around.