Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.
If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.
Use the XTGETTCAP sequence to determine if the host terminal supports
the OSC 52 sequence and, if it does, enable the OSC 52 clipboard
provider by default.
This is only done automatically when all of the following are true:
1. Nvim is running in the TUI
2. 'clipboard' is not set to unnamed or unnamedplus
3. g:clipboard is unset
4. Nvim is running in an SSH connection ($SSH_TTY is set)
5. Nvim is not running inside tmux ($TMUX is unset)
