Eglot: try reuse file watchers when server over-watches

Eglot: try reuse file watchers when server over-watches

GitHub-reference: https://github.com/joaotavora/eglot/pull/1228
GitHub-reference: https://github.com/joaotavora/eglot/discussions/1226

The Pyright language server issues very heavy file watching requests,
which sometimes exceed the OS limit.  Most of these file watches are
useless, but Pyright insists on issuing them.

What's more, for some (absurd?) reason, Pyright issues two file
watching requests for the _same_ directories, only to then almost
immediately ask to undo the effects of one of these requests.

This change to Eglot makes it so that if a single server requests to
watch a specific directory twice, only one file watch object is used.

Suggested by: https://github.com/thejeffphil

* lisp/progmodes/eglot.el (eglot-lsp-server): Change structure of
file-watches field.
(eglot--on-shutdown): Adapt to new structure.
(eglot-register-capability): Rework.
(eglot-unregister-capability): Rework.

* etc/EGLOT-NEWS: Mention change