[LSP] Document highlighting throws exception with F# files

[allisonchou]

Since syntax trees aren't supported in F#, this line throws an exception upon opening a default F# console app: https://github.com/dotnet/roslyn/blob/2297cd72e2224d140ba639d75432ddd0b0b5f0e7/src/Features/LanguageServer/Protocol/Handler/Highlights/DocumentHighlightHandler.cs#L72

cc @dibarbet

[CyrusNajmabadi]

This should likely be using IDocumentHighlightsService

[jasonmalinowski]

@dibarbet Did we already fix this? I assume we'd be getting lots of reports if we hadn't.

[dibarbet]

@jasonmalinowski afaik this feature isn't turned on unless the LSP editor feature flag is turned on, so users shouldn't be hitting this. That being said I haven't tried myself so I will confirm.