Close #770: Mark eglot-completion-at-point capf "non-exclusive"

Add :exclusive 'no to eglot-completion-at-point results marking it as
non-exclusive. This will allow completion to fall back to other less precise
completion backends (e.g. dabbrev) if Eglot's returns no results.

* eglot.el (eglot-completion-at-point): Set :exclusive to 'no

Copyright-paperwork-exempt: Yes

eglot.el

@@ -2415,6 +2415,7 @@ is not active."
             (regexp-opt
              (cl-coerce (cl-getf completion-capability :triggerCharacters) 'list))
             (line-beginning-position))))
+       :exclusive 'no
        :exit-function
        (lambda (proxy status)
          (when (eq status 'finished)