feat(tui): restore 'ttyfast' to control tty requests
Problem:
When running nvim on a remote machine over SSH, if there is high ping,
then bg detection may not complete in time. This results in a warning
every time nvim is started. #38648
Solution:
Restore 'ttyfast' option and allow it to control whether or not bg
detection is performed. Because this is during startup and before any
user config or commands, we use the environment variable
`NVIM_NOTTYFAST` to allow disabling `ttyfast` during initialization.
Co-authored-by: Kyle <50718101+kylesower@users.noreply.github.com>
