[web] Add a function for generating random strings

Summary:
Add a function for generating random strings on web. Add tests for the function.
This function will be used for generating device ID on web, issue: https://linear.app/comm/issue/ENG-1279/initialize-deviceid-on-clients

Test Plan: Run provided tests.

Reviewers: tomek, max, atul

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4731