dev: allow editing policies in Regal bundle without rebuilding Regal

[anderseknert]

This doesn't really apply to regal lint, as that's a one-off process anyway. But as more and more logic of the language server is moved from Go to Rego, it's becoming apparent that having to rebuild Regal and restart the language server (by restarting the editor) isn't as nice experience as it could be.

Famous last words for sure, but I think the solution could be simple. Enable "dev mode" by means of an environment variable, or regal language-server --dev flag, that when enabled does not source policies from the embedded Go code, but rather from the bundle directory on disk. This would be less performant for sure, but I think we can live with a hit to that if it means we get to see our changes as we work on them.