Use `zephir` as semantic identifier

In TextMate grammars, it's good style to mark all selectors with semantic identifiers for the language being highlighted.

Since you based off this TM grammar off the Javascript grammar, all the selectors end in `.js`, which is confusing. Even worse, the global `scopeName` is set to `source.js`, which can be an issue when loading the PLIST in many TextMate-compatible parsers.

This PR fixes the semantic identifiers. There should be no visible changes to the highlighting. :)