language_server: Consider exposing `report-unused-disable-directives` as a flag for the language server

[nrayburn-tech]

Might be nice to have this information exposed as a diagnostic from the language server. Users that use this option in the CLI would then be able to see the same errors in their IDE.

Should probably also support the severity variant as well, report-unused-disable-directives-severity.

[Sysix]

Is available in the next release 🥳
Sadly, no code action for removing it :/
I will look into it.