Allow custom package managers

[NotNite]

Right now, we just npm i -g pnpm in the Docker container, which is bad. We aren't pinning a version, it's slow as hell for every build, and it locks moonlight extension developers into using only pnpm.

It may be worth to use something like Corepack, which wraps package managers into one unified command line tool. Disclaimer: I have never used Corepack before, I've only read about it. I don't know how Corepack works for existing repositories (which don't specify a package manager), so that'll need to be looked into.

[TheEssem]

Corepack creates shims for the package managers and manages their different versions/uses them automatically, as far as I'm aware it does not provide a way to invoke them in a generic manner. If a packageManager field is specified in the JSON, it will automatically download, install, and use that version if you run the corresponding package manager like usual; if it's not specified, it will attempt to use the latest version of the package manager being invoked.

I'd probably suggest parsing the JSON to check for a packageManager field and run the one specified there (Corepack should use the correct version automatically), and if the field is missing then you could probably try searching for a lockfile.