[lib] Introduce basePluralize

Summary:
In the next diff, I introduce a function that pluralizes EntityText. This diff factors out the shared functionality.

Depends on D6510

Test Plan: The test cases in lib/utils/text-utils.test.js are extremely thorough

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6511