focusAtom and Setter optic

[lukeKonopka]

It would be nice if focusAtom worked with Setter type from optics-ts.
Using it with focusAtom should produce a write-only atom.
One possible use case would be to allow for appending to an array inside an atom, like this:

const stringArrayAtom = atom<string[]>([]);

const appendToArrayAtom = focusAtom(stringArrayAtom, (optic) => optic.appendTo()); // WritableAtom<null, [SetStateAction<string>], void>

As far as I can tell, it should work with the current implementation, we only need to change typing for focusAtom to accept Setter optic and return corresponding WritableAtom.

If it sounds like something worth supporting, I can create a PR with that change.

[dai-shi]

It sounds worth supporting to me.
cc: @merisbahti @arjunvegda

[arjunvegda]

+1 That would be a good addition to the library!

Would Getters be a good addition, too, or should we prefer read-only atoms instead?