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').
Note: formatc.lua was unable to swallow some newer additions to ExprASTNodeType
(specifically `kExprNodeOr = '|'` and probably something else), so all `= …`
were dropped: in any case they only were there in order to not bother updating
viml_pexpr_debug_print_ast_node and since it is now known all nodes which will
be present it is not much of an issue.
Currently supported nodes:
- Register as it is one of the simplest value nodes (even numbers are
not that simple with that dot handling).
- Plus, both unary and binary.
- Parenthesis, both nesting and calling.
Note regarding unit tests: it stores data for AST in highlighting in
strings in place of tables because luassert fails to do a good job at
representing big tables. Squashing a bunch of data into a single string
simply yields more readable result.
