This updates the docs and the driver, in a manner backwards compatible with
existing highlighters.  (None of the highlighters are touched by this change,
yet tests continue to pass.)

Part of issue #329.

Suggested-by: Matthew Martin

Makes the text easier to read ("_zsh_highlight_myhighlighter_highlighter" is
a mouthful).

Follow-up to 2176692704.

Part of #306.

Part of #306

The preceding commit, which made docs/highlighters.md and
highlighters/README.md distinct, allows reverting their removal.

* 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