Slim template language syntax highlighting

[detomastah]

Adds support for highlighting Slim templates: https://github.com/slim-template/slim (popular in Ruby on Rails)

Adapted from TextMate syntax highlighting. Proof that it works:

[jgmdev]

This new language syntax should be registered on the manifest.json file. Also, are the underscores between current_theme_mode made a different symbol type on purpose?

Edit: about the underscores

Maybe adding a _ after the %w would help here:

      -- Generic identifiers (best-effort fallback).
      -- Matches:
      --   div
      --   render
      pattern = "[%a][%w]*",