chore: make TestWatcher extend emittery

[SimenB]

Summary

This gives us typed events. I'm not sure if this should be considered a breaking change or not?

Test plan

Green CI

[thymikee]

Can we vendor the typings from emittery or it's not too productive because the package is small or so? I wouldn't consider this a breaking change, because it only affects small part of TS users (watch plugins builds maybe?), and TS tend to break between minor versions.

[SimenB]

Can we vendor the typings from emittery or it's not too productive because the package is small or so?

It'll come into the dep tree via #10227 regardless.

I wouldn't consider this a breaking change, because it only affects small part of TS users

Events are always emitted async now, not sure if it matters, tho.

No issue saving this for 27 to be on the safe side

[github-actions]

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