Work on https://github.com/neovim/neovim/issues/459

"cfuncs" was only ever used to wrap luarefs. As vim8script is
finished and will not be developed further, support for "cfuncs"
for other usecases are not planned. This abstraction was immediately
broken anyway in order to get luarefs out of userfuncs again.

Even if a new kind of userfunc needs to be invented in the future,
likely just extending the FC_... flag union directy, instead of
invoking unnecessary heap object and c function pointer indirection,
will be a more straightforward design pattern.

Work on https://github.com/neovim/neovim/issues/459

- Added 'spell' option to extmarks:

  Extmarks with this set will have the region spellchecked.

- Added 'noplainbuffer' option to 'spelloptions':

  This is used to tell Neovim not to spellcheck the buffer. The old
  behaviour was to spell check the whole buffer unless :syntax was set.

- Added spelling support to the treesitter highlighter:

  @spell captures in highlights.scm are used to define regions which
  should be spell checked.

- Added support for navigating spell errors for extmarks:

  Works for both ephemeral and static extmarks

- Added '_on_spell_nav' callback for decoration providers:

  Since ephemeral callbacks are only drawn for the visible screen,
  providers must implement this callback to instruct Neovim which
  regions in the buffer need can be spell checked.

  The callback takes a start position and an end position.

  Note: this callback is subject to change hence the _ prefix.

- Added spell captures for built-in support languages

Co-authored-by: Lewis Russell <lewis6991@gmail.com>
Co-authored-by: Björn Linse <bjorn.linse@gmail.com>

fix(api)!: correctly deal with number before :tab

Now nvim_parse_cmd and nvim_create_user_command use a "tab" value which
is the same as the number passed before :tab modifier instead of the
number plus 1, and "tab" value is -1 if :tab modifier is not used.

refactor(highlight): make hlattrs2dict always use pre-allocated dict