Some emacs users can open many buffers at a time.
Packages like `desktop.el` and `persp-mode` encourage this.
A user may also invoke eglot-ensure when each buffer starts.

The effect of this - spawning hundreds of language-server processes -
can crash the server and lag the system.

Now, `eglot-ensure` waits until window-configuration-change time
before starting `eglot--managed-mode`. Window-configuration-change
time is when the user interacts with the local buffer's window UI.

Used João Távora's idea to apply a pattern from flymake:
https://github.com/emacs-straight/flymake/blob/6bc8782d9c05d4c9bbba524542d457e6a3b1734b/flymake.el#L1013

Copyright-paperwork-exempt: Yes