Add module type declaration for makeup-focusables

[caseycarroll]

What

Introduce module type declaration file for makeup-focusables.

Why

TS consumers of makeup-focusables must add an ambient module declarations to their project to use makeup-focusables without TS errors.

I'm open to submitting this PR, if needed. I assume the team may want to add type declarations for all modules rather than just one. I'm happy to do that as well.