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.

Use the XTGETTCAP sequence to determine if the host terminal supports
the OSC 52 sequence and, if it does, enable the OSC 52 clipboard
provider by default.

This is only done automatically when all of the following are true:

  1. Nvim is running in the TUI
  2. 'clipboard' is not set to unnamed or unnamedplus
  3. g:clipboard is unset
  4. Nvim is running in an SSH connection ($SSH_TTY is set)
  5. Nvim is not running inside tmux ($TMUX is unset)