chore: improve ExpectionAssertion type in Brittle TypeScript definitions

[achou11]

Noticed a couple of minor issues:

• types were not handling providing a promise (note, not a function returning a promise) as the first argument
• was noticing some misleading "unnecessary awaits" warnings in my editor, even though the provided function was async. narrowed it down to needing to adjust the order of the overload so that it was ordered from most specific to least specific (i.e. () => Promise<unknown> is a subset of () => unknown)

[achou11]

technically fn is kind of a misleading parameter name in the case of providing a promise, but didn't think that creating another overload made sense vs using a union

[EvanHahn]

Thanks for fixing this!