Test against Typescript nightlies

[cjquines]

We should maybe start typechecking against the Typescript nightlies, or at least the betas, so we can get early signals; I know that type-fest does this for example.

Anyway, the only breaking change for us is regex checking, which helpfully informs us that the u regex flag is only available when targeting es6 or later. Should we just remove it in master?

[eranhirsch]

We probably won't officially support Typescript 5.5 in V1, and in V2 we compile with target ES2022. Would that allow us to avoid handling anything for typescript 5.5?

Let's create a separate issue (or use this one) for your suggestion on testing against nightly.

[cjquines]

yep, v2 is clean against 5.5. i'll edit this issue for testing against nightly