ASP.NET Razor is prototyping a system where they can add files into
our workspace via IDynamicFileInfoProvider and can then query our LSP
server to get diagnostics about those files. They're normally closed
files but we want them to get analyzed always if they want to query
them via LSP, and we also don't want those diagnostics to end up in the
error list since further processing needs to be done with them with
LSP.

This implements a hook where Razor can specify that diagnostics produced
for their documents are ingored by most of the system, but can still
be queried through their special LSP server.

Revert moving FAR to ILanguageClient.  We cannot safely use JTF to switch to the main thread across streamjsonrpc.