* build(contrib): add zsh completion

this is not part of the build system yet, so packager managers are
supposed to install the file by themselves.

bash doesn't seem to provide shell completion, zsh embeds its own
completion that is bundled with vim's.

Instead of copying zsh's completion, this generated one via
https://github.com/RobSis/zsh-completion-generator


---------

Co-authored-by: Eisuke Kawashima <e.kawaschima+github@gmail.com>

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
...
```

Problem:
`termopen` has long been a superficial wrapper around `jobstart`, and
has no real purpose. Also, `vim.system` and `nvim_open_term` presumably
will replace all features of `jobstart` and `termopen`, so centralizing
the logic will help with that.

Solution:
- Introduce `eval/deprecated.c`, where all deprecated eval funcs will live.
- Introduce "term" flag of `jobstart`.
- Deprecate `termopen`.

- 'statuscolumn' is no longer experimental
- add tags for popular searches on neovim.io

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.

It does not work and we don't plan on maintaining these anymore.

The flake files are being moved to
https://github.com/nix-community/neovim-nightly-overlay/pull/516
instead.

- Updates nixpkgs to source a necessary Tree-sitter version
- Updates to a new llvm version as the old one was removed from nixpkgs
- Properly moves `doCheck` from a regular param to an attribute option

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