tmux intercepts and ignores XTGETTCAP so wrap the query in the tmux
passthrough sequence to make sure the query arrives at the "host"
terminal.
Users must still set the 'allow-passthrough' option in their tmux.conf.
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)
