Support optional diagnostic.tags

Support optional diagnostic.tags

https://microsoft.github.io/language-server-protocol/specifications/specification-3-17/#diagnosticTag

A DiagnosticTag can be either 1 (DiagnosticTag.Unnecessary) or
2 (DiagnosticTag.Deprecated).  Following the rendering suggestions in
the protocol, we fade out Unnecessary code and strike-through
Deprecated code.

* eglot.el (eglot-diagnostic-tag-unnecessary-face)
(eglot-diagnostic-tag-deprecated-face): New faces.
(eglot--tag-faces): New defconst.
(eglot--lsp-interface-alist): Add Diagnostic.tags.
(eglot-client-capabilities): Advertise supported tags.
(eglot-handle-notification): Assign the appropriate properties.

* eglot-tests.el (diagnostic-tags-unnecessary-code): New test.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/794