🚀 Feature: Re-run only affected modules in watch mode

[boneskull]

This would really help teams that have very large test suites.

I am not sure exactly how other test frameworks accomplish this, but I can think of a few strategies.

@craigtaub if you're looking for something to do, researching various strategies and proposing a way forward would be helpful.

When we do implement, we can assume that the computation of the "affected module hierarchy" will save time over just running everything (as we currently do), so we can probably avoid worrying too much about caching in an MVP.

[boneskull]

@craigtaub if you want to look at this, please assign to yourself

[craigtaub]

Sounds interesting. Will look into this when I get some time.

[JoshuaKGoldberg]

Related: #4350

[JoshuaKGoldberg]

Marking as accepting PRs. But as with https://github.com/mochajs/mocha/issues/4350#issuecomment-668721125: this is not going to be easy. It's a big feature ask. I'd strongly suggest anybody looking at this issue to get through multiple other issues first before trying this. 🙂