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()`.

Problem:
The `termcap.lua` module is too narrowly named. We may need a place for
tty-related functionality in the future.
https://github.com/neovim/neovim/pull/31399#discussion_r1882598297

Solution:
This isn't a public/documented interface, so just rename it.

Problem:
In autocmd examples, using "args" as the event-object name is vague and
may be confused with a user-command.

Solution:
Use "ev" as the conventional event-object name.

This function allows the Nvim core to write arbitrary data to a TTY
connected to a UI's stdout.

Many terminals now include support for OSC 52 in their Primary Device
Attributes (DA1) response. This is preferable to using XTGETTCAP because
DA1 is _much_ more broadly supported.

Problem:

Nvim tries to use OSC 52 even when no TUIs are attached.

Solution:

On each UIEnter/UILeave event, check that there is a TUI client connected to Nvim's stdout.

We currently enable the OSC 52 clipboard provider by setting g:clipboard
when a list of conditions are met, one of which is that $SSH_TTY must be
set. We include this condition because often OSC 52 is not the best
clipboard provider, so if there are "local" providers available Nvim
should prefer those over OSC 52.

However, if no other providers are available, Nvim should use OSC 52
even when $SSH_TTY is not set. When a user is in an SSH session then the
checks for the other clipboard providers will still (typically) fail, so
OSC 52 continues to be enabled by default in SSH sessions.

This is marked as a breaking change because there are some cases where
OSC 52 wasn't enabled before and is now (or vice versa).

Problem: Using 'vim.iter' loads it during startup.

Solution: Do not use 'vim.iter'.

Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.

If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.

tmux has a set-clipboard option which, when set to 'on', allows
applications to set the system clipboard using the usual OSC 52 escape
sequence.