This adds support for the secondary DA escape sequence response.
Alacritty's version is formatted allowing for up to 99 minor and patch
versions, which should be sufficient.
The tertiary DA is intentionally not implemented and marked as rejected
in the documentation, since a lot of terminals do not support it, or
report useless data (XTerm/URxvt/Kitty).
Fixes#3100.
This doc was found in the tmux repository, and I'm adding it here so it
doesn't get lost. Having it in-tree will also allow it to be referenced
from the code.
