Add language_zsh syntax highlighting for Zsh

[merlindiavova]

• Recognises common Zsh files like .zshrc, .zshenv, .zprofile, etc.
• Built off the language_sh syntax plugin.

[jgmdev]

if we are going to have a dedicated language plugin for zsh then .zsh extension should be dropped from language_sh to prevent it from stepping over language_zsh

Edit: noticed that the language file has mixed indentation types, tabs and spaces. Also, besides .zshrc, .zshenv, .zprofile are there any special patterns? if not, maybe it would be better to just add those extensions to language_sh saw the additional keywords and some other differences.