bug#68214: Completion sorting customization by category

[Daniel Mendler]

Hi Juri,

thank you for this patch. Would it make sense to define a general
`completion-metadata-override-get` function instead of
`completion--display-sort-function`? This function could be used to look
up the other meta data functions too, `display-sort-function`,
`annotation-function`, `affixation-function`, `group-function`, etc.

(defun completion-metadata-override-get (metadata prop)
  (if-let ((cat (completion-metadata-get metadata 'category))
           (over (completion--category-override cat prop)))
      (cdr over)
    (completion-metadata-get metadata prop)))

I suggest to use `if-let` instead of `let*`, such that an override is
not retrieved if the category is nil.

Besides that, in the `completion-category-overrides`, you use
`completion--sorting-type` for the `display-sort-function`, while it
should just be `function`.

Daniel