Problem:
`clint.py` is the last python in our codebase, and beyond that it needs
some cleanup. And it lacks tests, so modifying it can be painful.
Also, we need a way to add ad-hoc lint rules for *Lua*, so it will help
to have our ad-hoc rules for C in the same language (the scripts may
share functions/techniques): https://github.com/neovim/neovim/issues/28372
Solution:
- convert to `clint.lua` (mostly AI-generated, but it now has test
coverage, unlike `clint.py`)
- drop rules that are no longer needed:
- "readability/multiline_string"
- technially still relevant, but very uncommon so doesn't really matter.
- "--line-length"
- Not used in the old clint.py, nor the new clint.lua.
- "comment whitespace" check
- It is enforced by uncrustify.
- "TODO" check
- The `-google-readability-function-size` clang-tidy rule enforces
"TODO(user)" format. (It was already enabled long ago.)
