Remove syncolor.vim in favor of defining the default highlight groups
directly in `init_highlight`. This approach provides a number of
advantages:

1. The highlights are always defined, regardless of whether or not the
   syntax regex engine is enabled.
2. Redundant sourcing of syntax files is eliminated (syncolor.vim was
   often sourced multiple times based on how the user's colorscheme file
   was written).
3. The syntax highlighting regex engine and the highlight groups
   themselves are more fully decoupled.
4. Removal of the confusing `:syntax on` / `:syntax enable` dichotomy
   (they now both do the same thing).

This approach also correctly solves a number of bugs related to
highlighting (#15176, #12573, #15205).

luassert uses 3 by default, which is often not enough.

Instead of documenting how to increase it, let's use a more fitting
(sane) default of 100 levels.

The test.functional.helpers and test.unit.helpers modules now include
all of the public functions from test.helpers, so there is no need to
separately require('test.helpers').

It appears to be different on lua and luajit.

With the new test leaving `assert(false);` for any of the cases makes tests 
crash.