Problem:
`vim.validate()` takes two forms when it only needs one.
Solution:
- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
of the `optional` argument.
If multiple XTGETTCAP requests are active at once (for example, for
requesting the Ms capability and truecolor capabilities), then the
TermResponse autocommand may fire for capabilities that were not
requested. Instead, make sure that the provided callback is only called
for capabilities that were actually requested.
Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.
If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.
tmux intercepts and ignores XTGETTCAP so wrap the query in the tmux
passthrough sequence to make sure the query arrives at the "host"
terminal.
Users must still set the 'allow-passthrough' option in their tmux.conf.
Use the XTGETTCAP sequence to determine if the host terminal supports
the OSC 52 sequence and, if it does, enable the OSC 52 clipboard
provider by default.
This is only done automatically when all of the following are true:
1. Nvim is running in the TUI
2. 'clipboard' is not set to unnamed or unnamedplus
3. g:clipboard is unset
4. Nvim is running in an SSH connection ($SSH_TTY is set)
5. Nvim is not running inside tmux ($TMUX is unset)
