Enable strict typing in Formula

[dduugg]

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[p-linnane]

😍

[MikeMcQuaid]

Looks good to me once 🟢! Feels almost certain to go 💥 a bit after merge so let's wait until not too near to a (Monday, usually) release tag and be around for a few hours "babysitting" if possible 🙇🏻

The only (optional/non-blocking) preference I'd have here is to use T.must less and instead do e.g. local assignment and early returns because it's a bit more clear and nicer, in my opinion.

[samford]

I opened #19338 to account for these nilable values instead of using T.must. It also adds type signatures to related methods in the process, so brew typecheck will identify this issue as expected (shout if any of it conflicts with this PR).

Edit: Now merged, so this can be addressed in a rebase.