Eglot: ensure quick typing doesn't undermine completion exit

Eglot: ensure quick typing doesn't undermine completion exit

Some completion resolution requests in
eglot-completion-at-point, specifically those that might be
triggered from the :exit function, can't be cancellable, else
the completion edit or snippet will never be obtained and the
user is left with the nonsensical completion label in the
buffer.

Github-reference: https://github.com/joaotavora/eglot/issues/1474
(cherry picked from commit b71ddaf6d3ac39487302fbc27614b9de28bf6cef)