Eglot: try even harder to avoid other completion styles (bug#68699)

Eglot: try even harder to avoid other completion styles (bug#68699)

Any completion style except for eglot--dumb-flex spells trouble for
Eglot, for the well known reason that LSP is geared towards completion
tooltips and none of Emacs' partial-completion shenanigans.

This commit puts a "try-completion" function that doesn't return nil
in the eglot--dumb-flex completion style so that other styles aren't
tried (partial-completion, in particular, errors out).

The function often doesn't do anything very useful, but at least it
doesn't stop the more usual *Completions* buffer from appearing.

* lisp/progmodes/eglot.el (eglot--dumb-tryc): New helper.
(completion-styles-alist): Add it to the dumb-flex style.

(cherry picked from commit 65ea742ed5ec505837706d64690e3cc2073825c7)