Fix Eglot's snippet insertion to follow the manual

Fix Eglot's snippet insertion to follow the manual

The manual states that YASnippet must only be installed to be useful.
Before this change, it would only work if the user happened to have
manually activated it before with 'yas-global-mode' or somesuch.

This makes Eglot's Yasnippet-activating behaviour similar to its
Flymake-activating behaviour.

* lisp/progmodes/eglot.el (eglot-client-capabilities): Consult
eglot--stay-out-of.
(eglot--snippet-expansion-fn): Turn on yas-minor-mod eon demand.
(eglot-completion-at-point): Simplify.