Eglot: respect completion sort order dictated by the server

Eglot: respect completion sort order dictated by the server

Don't use flex style to do any completion sorting.

Previously, it was thought that the 'flex' completion style was only
kicking in to do (approximate) fontification of the completions
returned by the server, but it was found that it was also doing some
its own sorting in certain situation of non-empty matching patterns.

Replaced it with a new eglot--dumb-flex style which does only
fontification.

Github-reference: https://github.com/joaotavora/eglot/discussions/1306

* lisp/progmodes/eglot.el (eglot-completion-at-point): Rework.
(eglot--dumb-flex, eglot--dumb-allc): New helpers.
(completion-category-defaults): Rework Eglot-specific category.
(completion-styles-alist): Add Eglot-specific style.

* etc/EGLOT-NEWS: Mention change.