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

Special handling for cursor imprint or partial path highlighting
is needed in more cases than accept-*. For example when accepting
a line from isearch, no accept-* widget is invoked.

The proper way is to use zle-line-finish.

Trumps #259.
Fixes #284.

_zsh_highlight_add_highlight appends to region_highlight $1 $2 and the
next non-null parameter. If there is no non-null parameter, do nothing,
This is so that highlighters can add a style with fallbacks if the most
specific style is not defined. If none of the applicable styles are
defined, do the right thing and don't add an invalid entry to
region_highlight.

The pattern highlighter doesn't use this function as it'd need too large
of an API change.

Fixes zsh-users/zsh-syntax-highlighting#109.

Similar to 59fbdda64c in the 'main' highlighter.

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

* 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

Feel free to add more test data !

Signed-off-by: Julien Nicoulaud <julien.nicoulaud@gmail.com>