Proposal: switch-expression support

[koblas]

This is a pull request to add support for switch expressions #9603 please see the issue for more details.

[haltcase]

This is great and all but this will never be merged without an official proposal to TC39 (as much as I want a pattern matching expression 😆), as @nicolo-ribaudo pointed out in the linked issue.

#9603 (comment)

[koblas]

I've been working on a TC39 proposal in parallel to this PR. I wanted to provide a clear place to have an implementation where people could add some comments as needed.

When I look back at the pattern matching PR list, I see they did at least 3 PRs over time.

Is their a rule for babel TC39 proposals that they will only be consider if they're at least stage 1?

[nicolo-ribaudo]

Generally we give precedence to proposals which are in higher stages, but the only real requirement is that it is at least stage 1.

I'm closing this PR for now, but we can talk about it again when there is an official proposal considered by TC39.