JS prefixing check

[dingo-d]

Building on #907, a similar check should be made for JavaScript functions.

Atm the only thing that I can think of is creating a custom lint, that should be then added along with the WPTRT 120 issue, for general line check.

[jrfnl]

@dingo-d FYI: PHPCS already contains a tokenizer for JS as well. It's not as fully featured as the PHP one, but I got a suspicion that it shouldn't be hard to create a "normal" PHPCS sniff for this (or add to the PrefixAllGlobals one).

[jrfnl]

With an eye on PHPCS dropping support for JS in PHPCS 4.0 and most of the JS checking having moved to ESLint now anyway, I suggest closing this as wontfix.