Support didchangewatchedfiles with dynamic registration

Support didchangewatchedfiles with dynamic registration

RLS uses this, presumaly for knowing about Cargo.toml changes and stuff.

* README.md: Update protocol compliance.

* eglot.el (filenotify): Require it.
(eglot--file-watches): New process-local var.
(eglot--process-sentinel): Kill all watches
(eglot--register-unregister): New helper.
(eglot--server-client/registerCapability): Simplify.
(eglot--server-client/unregisterCapability): New method.
(eglot--register-workspace/didChangeWatchedFiles)
(eglot--unregister-workspace/didChangeWatchedFiles): New
capability.
(eglot--client-capabilities): Update.