fix(highlight): remove syncolor.vim

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).

src/nvim/main.c

@@ -372,11 +372,6 @@ int main(int argc, char **argv)
 
   // If using the runtime (-u is not NONE), enable syntax & filetype plugins.
   if (params.use_vimrc == NULL || !strequal(params.use_vimrc, "NONE")) {
-    // Source syncolor.vim to set up default UI highlights if the user didn't
-    // already enable a colorscheme
-    if (!get_var_value("g:colors_name")) {
-      source_runtime((char_u *)"syntax/syncolor.vim", DIP_ALL);
-    }
     // Does ":filetype plugin indent on".
     filetype_maybe_enable();
     // Sources syntax/syntax.vim, which calls `:filetype on`.