Add ```serverCancelled``` support for ```textDocument/documentSymbol```

[rtetley]

Problem

I am working on developing a language server for the rocq prover, https://github.com/coq/vscoq.
I have recently run into a problem with the document outline (in vscode).
In a recent change, I parse the document incrementally, using events that get popped up in an event loop. Many times the textDocument/documentSymbol request gets sent before the parsing of the document is finished.

Tried solutions

The first solution I explored was using progress based. Unfortunately it seems that vscode does not send a partialResultToken or a workDoneToken in the request although I did opt-in through the capabilities. Maybe there is a subtlety I do not understand ?

The second solution I explored comes from the disccusion in #1367 where it is suggested that after receiving a ServerCancelled error code the client well then re-send the request later. This does not seem to be happening for the documentSymbol request.

After some digging in the spec, it seems documentSymbol is not cancellable ? Compared to the semanticTokensClientCapabilities it does not contain the serverCancelledSupport field.

Proposed solution

Could we add serverCancelled support to the documentSymbol request ?

Related

Related to #1572 and #1367.

Also related to microsoft/vscode#135453