Problem:
- Windows users can't use terminfo to configure their terminal
capabilities. #37274
- Terminfo definitions sometimes get out of date or are simply
inaccurate.
- Eventually, we may want to drop terminfo, relying primarily on
built-in definitions. Users will still need some flexibility.
Solution:
Support $NVIM_TERMDEFS environment variable, which is JSON data that
defines "terminfo" definitions that override our builtin terminfo.
Problem:
- Various `TermRequest` handlers which all do similar things.
- `tty.query` is specific to `XTGETTCAP DCS`, can't be reused for other kinds of terminal queries.
Solution:
Provide `tty.request()`.