Create infrastructure for experimental branches

[DanielRosenwasser]

We'd like to create a vehicle for experimental branches that get merged into a temporary branch based off of master on a nightly basis, and published to npm. Presumably it'd be called something like typescript@experimental.

Users can use this branch to more-easily play with experimental features. This will allow us to keep branches in a reasonable state as they wait for the "feature" period of our release cycle, and will allow us to see how those features combine and interact.

This is a partial alternative to #28152 - the main difference being that

• this allows features to not be merged into master
• experimental features aren't flagged because they're not even part of nightly

[weswigham]

FYI, the infra I actually already put in place mostly, I just need to prep and merge things into an experimental branch to test.

[DanielRosenwasser]

Great! But now you have an issue to track the work too 💯👍🏻💯🔥👌🏻💯👍🏻💯🔥👌🏻

[weswigham]

https://typescript.visualstudio.com/TypeScript/_build?definitionId=31
https://github.com/typescript-bot/TypeScript/tree/experimental

You'd like to see said experimental branch published, yes?

[DanielRosenwasser]

Yes, every night pls

[weswigham]

Opened #32411 and #32412 based on our discussion earlier to better service our prerelease build needs - I'll decommission the experimental branch build that's already been running once that's together.