Fixes #418 (interoperability issue with other plugins).

This tracks the API change made in the previous commit, as suggested in the
(#if 0'd) deprecation warning.

The following warnings are issued by zsh as of zsh-workers/37018 (commit
de9effbce601 to zsh itself):

_zsh_highlight_main_highlighter_highlight_dollar_string:17: array parameter match created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_dollar_string:17: array parameter mbegin created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_dollar_string:17: array parameter mend created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_dollar_string:17: numeric parameter parameter MBEGIN created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_dollar_string:17: numeric parameter parameter MEND created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_dollar_string:17: scalar parameter MATCH created globally in function _zsh_highlight_main_highlighter_highlight_dollar_string
_zsh_highlight_main_highlighter_highlight_string:11: array parameter match created globally in function _zsh_highlight_main_highlighter_highlight_string
_zsh_highlight_main_highlighter_highlight_string:11: array parameter mbegin created globally in function _zsh_highlight_main_highlighter_highlight_string
_zsh_highlight_main_highlighter_highlight_string:11: array parameter mend created globally in function _zsh_highlight_main_highlighter_highlight_string
_zsh_highlight_main_highlighter_highlight_string:11: numeric parameter parameter MBEGIN created globally in function _zsh_highlight_main_highlighter_highlight_string
_zsh_highlight_main_highlighter_highlight_string:11: numeric parameter parameter MEND created globally in function _zsh_highlight_main_highlighter_highlight_string
_zsh_highlight_main_highlighter_highlight_string:11: scalar parameter MATCH created globally in function _zsh_highlight_main_highlighter_highlight_string

All these files should be sourced, not executed; and Debian's lintian complains:

W: zsh-syntax-highlighting: script-not-executable usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh

Signed-off-by: Takeshi Banse <takebi@laafc.net>

* Don't override user defined styles
* Better modularisation of highlighters
* Allow to define which highlighters are activated
* Allow to define the order in which they are defined
* Minor performance optimizations
* Fixed some variables leak
* Improve documentation
* Brackets highlighter: use ZSH_HIGHLIGHT_STYLES instead of a specific array