Allow completion frontends to fontify candidates just-in-time

Allow completion frontends to fontify candidates just-in-time

bug#48841, bug#47711

The variable may be bound by the frontend to a non-nil around
completion-producing calls like completion-all-completions.  See
completion-lazy-hilit docstring for more info.

* lisp/icomplete.el (icomplete-minibuffer-setup): Set completion-lazy-hilit.
(icomplete--render-vertical): Call completion-lazy-hilit.
(icomplete-completions): Call completion-lazy-hilit.

* lisp/minibuffer.el (completion-lazy-hilit): New variable.
(completion-lazy-hilit): New function.
(completion-lazy-hilit-fn): New variable.
(completion-pcm--regexp)
(completion--flex-score-last-md): New helper variables.
(completion--flex-score-1): New helper.
(completion-pcm--hilit-commonality): Use completion-lazy-hilit.
(completion--flex-adjust-metadata): Rework sorting code.

* etc/NEWS: Mention completion-lazy-hilit