[lib] Move MinimallyEncoded* validators to their own file

Summary:
This resolved circular dependency issue I experienced when trying to fix checkIfDefaultMembersAreVoiced.

Depends on D9848

Test Plan:
Flow/ESLint/Jest

IDE "Move" functionality handled this entire refactor, didn't do it by hand

Reviewers: ashoat, ginsu, rohan, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D9856