Unbreak m-x vc-revert, which reverts preserving modes

Unbreak m-x vc-revert, which reverts preserving modes

Unlike the normal revert-buffer command, vc-revert, doesn't re-apply
the major mode, meaning it was missing a didOpen to pair with the
didClose that is unconditionally sent on both commands.

Needed to use the dynamic variable revert-buffer-preserve-modes, and,
curiously, also forward-declare it to appease the byte compiler.

* eglot.el (eglot--managed-mode): Use after-revert-hook.
(revert-buffer-preserve-modes): Forward declare.
(eglot--after-revert-hook): Signal didOpen when preserving-modes.
(eglot--maybe-activate-editing-mode): Tweak comment.