Ref:
https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h3-Other-buttons

Also remove British National Replacement Character Set.

We keep the DEC Special Graphics and ASCII despite it not being unicode
as some old software such as calcurse still rely on this functionality.

References:
- https://github.com/neovim/neovim/pull/31934#discussion_r1911046426
- https://en.wikipedia.org/wiki/DEC_Special_Graphics
- https://vt100.net/docs/vt220-rm/chapter2.html#S2.4.3

Arrow and function keys do not use CSI u with the kitty keyboard
protocol. For example, the Up arrow key uses CSI A, and the function
keys use a variety of different CSI sequences.

Until now, termkey only parsed subparams used by key events for CSI u
sequences. The result being that any key which did not use CSI u (e.g.
arrow and function keys) was being emitted twice by termkey since it was
not recognizing the separate press and release events.

This commit makes termkey also parse subparams for other key sequences
so that the release key events do not send duplicate keys.

Enable key event reporting in the kitty keyboard protocol. This causes
supporting terminals to send key events for presses, repeats, and key
releases. For now we ignore release events, but eventually we will
support users mapping those.

Problem:
Since 2a7d0ed6145bf3f8b139c2694563f460f829813a, build fails with glibc
version 2.28 / RHEL8 (where `termios.h` does not include unistd.h and is
therefore missing `_POSIX_VDISABLE`):

    …/src/nvim/tui/termkey/termkey.c: In function 'termkey_start':
    …/src/nvim/tui/termkey/termkey.c:516:31: error: '_POSIX_VDISABLE' undeclared (first use in this function)
      516 |         termios.c_cc[VQUIT] = _POSIX_VDISABLE;
          |                               ^~~~~~~~~~~~~~~
    …/src/nvim/tui/termkey/termkey.c:516:31: note: each undeclared identifier is reported only once for each function it appears in

Solution:
- Undo the `<termios.h>` change and mark the imports with `IWYU pragma: keep`.

Result of `make iwyu` (after some "fixups").

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:
write() can return -1 but is cast to unsigned type causing coverity
to detect possible overflowed integer

Solution:
Perform check to ensure all negative values are captured rather than
just -1 before casting to unsigned type

Termkey is abandoned and it's now our code, so there's no reason not to
treat it as such. An alternative approach could be to have a proper repo
that we maintain such as with unibilium, although with this approach we
can make a few assumptions that will allow us to remove more code.

Also eliminate duplicate code from both termkey and libvterm.