Auto-adapt completion preview background color

Auto-adapt completion preview background color

Teach Completion Preview mode to automatically remap its
faces such that the background color of the preview overlay
matches the background color of the buffer text that is
being completed.  Crucially, this resolves an issue where
the preview overlay didn't look nice with hl-line-mode on.

Also see related discussion in bug#71282.

* lisp/completion-preview.el
(completion-preview-adapt-background-color): New option.
(completion-preview--bg-color): New function.
(completion-preview--face-remap-cookie-jar): New variable.
(completion-preview--make-overlay): Use them.