This completion style can optionally modify a completion table's
metadata to use `prescient.el` sorting, including
`prescient-sort-full-matches-first`.

- Update the CHANGELOG.
- Update the README.

- Prescient changes:
  - Add faces `prescient-primary-highlight` and
    `prescient-secondary-highlight`. Add a function to apply them.
  - Add user options `prescient-highlight-completions` and
    `prescient-completion-enable-sort`.
  - Make `prescient-filter` work with completion tables by using
    `all-completions` instead of assuming lists.
    - `prescient-filter` now propertizes all returned candidates
      with the regexps prescient used and whether it ignored case.
      This is needed for sorting after filtering.
  - Add filtering functions `prescient-all-completions`,
    `prescient-try-completion`, which use `prescient-filter`.
  - Add sorting functions `prescient--completion-modify-sort`,
    `prescient-sort-full-matches-first`, and
    `prescient-completion-sort`.
  - Define a completion style `prescient` that uses those functions.
  - The completion style can modify unsorted tables to use
    `prescient-completion-sort`, which wraps `prescient-sort` and
    `prescient-sort-full-matches-first`.
    - This requires Emacs 27+.
    - This is off by default to more easily work with multiple
      completion UIs. Some UIs (such as Vertico and Corfu) allow
      setting the sorting function to use after filtering.  Setting
      that option to the function `prescient-completion-sort` works
      and applies to the output of all completion styles and backends,
      not just `prescient`.

- Selectrum Prescient changes:
  - Old faces are now aliases of the completion faces.
  - Since Selectrum seems to remove the text properties of candidates
    after filtering, we use a function `selectrum-prescient--refine`
    that combines `prescient-filter` with
    `prescient-sort-full-matches-first`.
  - Move `selectrum-prescient--highlight` into Prescient for the
    completion style as `prescient--highlight-matches`. Use that
    instead.

- Company Prescient changes:
  - Change `company-prescient-transformer` so that uses
    `prescient-completion-sort` instead of just `prescient-sort`. This
    should allow for moving full matches when using CAPFs and will
    always at least do `prescient-sort`.

See also the PR #125. For various discussions about implementing a
`prescient` completion style, see also #125, #120, #112, #89, #58, and
#54.