Add T_BACKTICK to JS tokenizer

[rebeccahum]

It would be nice if we can add T_BACKTICK in the JS tokenizer since template literals use backticks.

[gsherwood]

I'm planning on removing support for JS and CSS tokenizing in version 4, so I'm not making any changes to the existing feature set. I suggest looking into dedicated JS linting tools like eslint.

[michelgokan]

@gsherwood one can simply add <exclude-pattern>*\.(inc|css|js)</exclude-pattern> in the rulesets, but then Generic.Debug.ESLint will not be effective on js files anymore.