Problem:
RelWithDebInfo generates redundant flags:

    Compilation: /usr/bin/cc -O2 -g -Og -g

The `CMAKE_C_FLAGS_RELWITHDEBINFO` variable is being modified in a way
that caused duplicate `-Og` and `-g` flags to be added. The resulting
flags were `-O2 -g -Og -g`.

- `-Og` (Optimize for debugging) and `-O2` (Optimize for performance)
  are different optimization levels. We can't use both at once.
- The duplicate `-g` flag is redundant and no effect.

multiple -O flags has no effect for code, just redundant.

> If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Solution:
Adjust the flags to use the more appropriate `-O2 -g`.

    Compilation: /usr/bin/cc -O2 -g

BEFORE:

```
:verbose version
NVIM v0.11.0-dev-1443+ge00cd1ab40
Build type: RelWithDebInfo
LuaJIT 2.1.1734355927
Compilation: /usr/bin/cc -O2 -g -Og -g -flto -fno-fat-lto-ob
jects -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict
...
```

AFTER:

```
:verbose version
NVIM v0.11.0-dev-e00cd1ab4-dirty
Build type: RelWithDebInfo
LuaJIT 2.1.1734355927
Compilation: /usr/bin/cc -O2 -g -flto -fno-fat-lto-objects -
Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-protot
...
```

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Co-authored-by: Lauri Heiskanen <lauri.heiskanen@nimble.fi>
Co-authored-by: Piotr Doroszewski <5605596+Doroszewski@users.noreply.github.com>
Co-authored-by: Tobiasz Laskowski <tobil4sk@outlook.com>
Co-authored-by: ariel-lindemann <41641978+ariel-lindemann@users.noreply.github.com>
Co-authored-by: glepnir <glephunter@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem: Adding support for modern Nvim features (reflow, OSC 8, full
utf8/emoji support) requires coupling libvterm to Nvim internals
(e.g., utf8proc).

Solution: Vendor libvterm at v0.3.3.

Also add missing mention of libvterm and remove mention of libtermkey.

Problem:

The documentation flow (`gen_vimdoc.py`) has several issues:
- it's not very versatile
- depends on doxygen
- doesn't work well with Lua code as it requires an awkward filter script to convert it into pseudo-C.
- The intermediate XML files and filters makes it too much like a rube goldberg machine.

Solution:

Re-implement the flow using Lua, LPEG and treesitter.

- `gen_vimdoc.py` is now replaced with `gen_vimdoc.lua` and replicates a portion of the logic.
- `lua2dox.lua` is gone!
- No more XML files.
- Doxygen is now longer used and instead we now use:
  - LPEG for comment parsing (see `scripts/luacats_grammar.lua` and `scripts/cdoc_grammar.lua`).
  - LPEG for C parsing (see `scripts/cdoc_parser.lua`)
  - Lua patterns for Lua parsing (see `scripts/luacats_parser.lua`).
  - Treesitter for Markdown parsing (see `scripts/text_utils.lua`).
- The generated `runtime/doc/*.mpack` files have been removed.
   - `scripts/gen_eval_files.lua` now instead uses `scripts/cdoc_parser.lua` directly.
- Text wrapping is implemented in `scripts/text_utils.lua` and appears to produce more consistent results (the main contributer to the diff of this change).

Luarocks is no longer needed after
25e51d393a420765d5efd44c1b4be823a5cf280a.

Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>
Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>

Also update doc.