completion-boundaries support

[clemera]

See comment by @minad:

I tried your checko example from above again and it would actually nice if there would be some limited support for completion boundaries in Selectrum.

(defun grep-test (string pred action)
  (if (eq (car-safe action) 'boundaries)
      (cons 'boundaries
            (cons
             (if-let (pos (seq-position string ?#))
                 (1+ pos)
               (length string)) 0))
    (complete-with-action action '(alpha beta gamma) "" pred)))

If you enter grep#filter, then "grep" is ignored by completion and "filter" is used for completion. This allows also to TAB cycle through the results in default completion which is an improvement over the status quo in consult-grep.