Problem: buffer text with composing chars are converted from UTF-8
to an array of up to seven UTF-32 values and then converted back
to UTF-8 strings.

Solution: Convert buffer text directly to UTF-8 based schar_T values.

The limit of the text size is now in schar_T bytes, which is currently
31+1 but easily could be raised as it no longer multiplies the size
of the entire screen grid when not used, the full size is only required
for temporary scratch buffers.

Also does some general cleanup to win_line text handling, which was
unnecessarily complicated due to multibyte rendering being an "opt-in"
feature long ago. Nowadays, a char is just a char, regardless if it consists
of one ASCII byte or multiple bytes.

We already have an extensive suite of static analysis tools we use,
which causes a fair bit of redundancy as we get duplicate warnings. PVS
is also prone to give false warnings which creates a lot of work to
identify and disable.

refactor: use a more idiomatic loop to iterate over the cells

There are two cases in which the following assertion would fail:
```c
assert(g->icell < g->ncells);
```

1. If `g->ncells = 0`. Update this to be legal.
2. If an EOF is reached while parsing `wrap`. In this case, the unpacker
   attempts to resume from `cells`, which is a bug. Create a new state
   for parsing `wrap`.

Reference: https://neovim.io/doc/user/ui.html#ui-event-grid_line

Co-authored-by: Wansmer <wansmer@gmail.com>
Co-authored-by: Andrew Voynov <andrewvoynov.b@gmail.com>
Co-authored-by: David Moberg <david.moberg@mediatek.com>

- Move vimoption_T to option.h
- option_defs.h is for option-related types
- option_vars.h corresponds to Vim's option.h
- option_defs.h and option_vars.h don't include each other

Linux added these types to their userspace headers in [6.5], which
causes unit tests to fail like

```
-------- Running tests from test/unit/api/private_helpers_spec.lua
RUN       vim_to_object converts true: 17.00 ms ERR
test/unit/helpers.lua:748: test/unit/helpers.lua:732: (string) '
test/unit/helpers.lua:264: ';' expected near '__s128' at line 194'
exit code: 256

stack traceback:
	test/unit/helpers.lua:748: in function 'itp_parent'
	test/unit/helpers.lua:784: in function <test/unit/helpers.lua:774>
```

Since we don't use these types, they can be ignored to avoid LuaJIT's C
parser choking on them.

[6.5]: https://github.com/torvalds/linux/commit/224d80c584d3016cb8d83d1c33914fdd3508aa8c

Co-Authored-By: L Lllvvuu <git@llllvvuu.dev>