The data to be written can be very long, so use nvim_chan_send() instead
of io.stdout:write() as the latter doesn't handle EAGAIN.
A difference of these two approaches is that nvim_chan_send() writes to
stderr, not stdout, so it won't work if client stderr is redirected.
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)
Problem:
Platform-specific UI providers should live in `vim.ui.*`. #24164
Solution:
- Move `vim.clipboard.osc52` module to `vim.ui.clipboard.osc52`.
- TODO: move all of `clipboard.vim` to `vim.ui.clipboard`.
ref #25872
