imapclient: initial VANISHED support
v2: ran go fmt
This seems to overlap a bit with https://github.com/emersion/go-imap/pull/690.
I did a review of that branch (I admit I didn't see it before, because I looked for keyword VANISHED.
I only notice one difference, it uses:
UID FETCH 1:* (FLAGS) (CHANGEDSINCE 123) VANISHED
Whereas my branch uses:
UID FETCH 1:* (FLAGS) (CHANGEDSINCE 123 VANISHED)
I believe my approach to be correct. From: https://datatracker.ietf.org/doc/html/rfc7162#section-3.2.6:
The VANISHED UID FETCH modifier MUST only be specified together with the CHANGEDSINCE UID FETCH modifier.
I think it's probably best to fix that in https://github.com/emersion/go-imap/pull/690 and drop this branch.