Allow opting out of automatic language registration

[allejo]

• Fix Highlighter::listLanguages() not getting correctly updated when custom languages were registered
• Add new $loadAllLanguages parameter to Highlighter constructor
• Make Highlighter::registerAllLanguages() public
• Add Highlighter::clearAllLanguages() to allow clearing registered languages
• Add two new utility functions
  • getLanguagesFolder()
  • getLanguageDefinitionPath()
• Add new Highlighter::listBundledLanguages() function
• Add new Highlighter::listRegisteredLanguages() function
• Deprecate Highlighter::listLanguages(), use listBundledLanguages() or listRegisteredLanguages() instead

/cc @westonruter