BREAKING CHANGE: This commit changes the behaviour of the TAB and RET
keys in a consult-completing read multiple session, in order to make
them more intuitive. The behaviour is now:
- TAB: (unchanged) always select or deselect the current candidate, and
if the candidate is selected, move the index to the next one (this
allows for pressing TAB repeatedly to select multiple subsequent
candidates).
- RET: If no candidates have been selected, select the current candidate
and exit the completion session. If some have been selected, disregard
the current candidate and exit.
- S-TAB: (new) like TAB, but the keeps the input.
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