Score flex-style completions according to match tightness

Score flex-style completions according to match tightness

The new completion style needs to score completion matches so that we
can use it later on when sorting the completions.  This is because, in
the flex style, "foo" can now match "foobar", "frodo" and
"barfromsober" but we probably want "foobar" to appear at the top of
the completion list.

This change makes the new flex completion style add sort-order hints
under the completion string's `completion-style-sort-order' property.

* lisp/minibuffer.el (completion-pcm--hilit-commonality): Propertize
completion with 'completion-pcm-commonality-score.
(completion-flx-all-completions): Propertize completion with
completion-style-sort-order and completion-style-annotation.