REPL: please add pipeline proposal options: Hack, Elixir

[lightmare]

Currently it's kinda difficult to test experimental flavours in the REPL.

Alternatively, include the pipeline proposal value in URL. This would probably be even more useful, as besides allowing sending links with pipeline flavour pre-selected, it would also allow me to put an unlisted value in there.

[babel-bot]

Hey @lightmare! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
community that typically always has someone willing to help. You can sign-up here
for an invite.

[JLHwung]

You can draft an REPL PR to this repo. Netlify will host a PR website, where you can have REPL attached to the babel standalone built from your Babel PR.

[lightmare]

@JLHwung Thank you. Figuring out how to attach it to Babel PR took me a while (at first I only changed version= in the URL, which didn't do anything), but got it working in the end.

[nicolo-ribaudo]

@lightmare Are you interested in opening an alternative to #2553 that adds Hack and removes Smart (since Hack has been added in 7.15.0) so that we can merge it to the main repl? Otherwise I can do it.

[lightmare]

I'll take a look. There are some new conflicts I haven't seen.

[lightmare]

New PR #2563