refactor: clint.py => clint.lua

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.)

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