Only sends a `textDocument/documentHighlight` request when on an identifier

[Veykril]

VSCode doesn't seem to send a textDocument/documentHighlight request when the cursor isn't on an identifier(whatever this means to vscode). The LSP documentation doesn't state that this request should only be sent in that case though.

The document highlight request is sent from the client to the server to resolve a document highlights for a given text document position.
https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_documentHighlight

We ran into this in rust-analyzer when trying to highlight exit points, which in rust can have the form of a simple ? character. So having the cursor on this character currently can't trigger the highlight. rust-lang/rust-analyzer#9375

[Veykril]

Meant to open this in the vscode repo, sorry