move the password strength checker utility function in a "shared" folder that can be accessed by both frontend and backend

[bcha92]

Description

Pre-requisite task to complete first: #196

Currently, there are TWO password strength checkers, one in the BE api repo and the other in the FE src repo. They are literally duplicates of one another, and should be moved to a single shared folder that can utilize from both repos.

BONUS: Documentation and tests of the password strength checker and what it does.