Fix/Update bash language

This patch includes the following improvements to the bash language:

* Remove dependency on `clike` because it has almost nothing in common
  with it.
* Add some missing keywords and builtin functions
* Add support for switch statements
* Add support for arithmetic environments
* Allow +-*/ only in arithmetic environments
* Fix hashbang syntax
* Allow variables only in double quoted strings
* Use aliases for hashbang and variable-usage
* Highlight variable definitions