Add script for frontend test file coverage

What does this MR do?

Add a script that counts the number of frontend source files and compares it to the number of frontend test files.

It can be run with

node scripts/frontend/file_test_coverage.js

or

node scripts/frontend/file_test_coverage.js -v

to print the file names.

It's intended to help with gitlab-com/www-gitlab-com#4329 (closed).