feat: support cancellation of RPC requests

[Vtec234]

Adds support for cancelling LSP and RPC requests in infoview code (including widgets). No changes to Lean are needed since the server already handles $/cancelRequest, and RequestM includes checkCancelled.

Main changes:

• Add an optional AbortSignal argument to LSP and RPC calls. Aborting causes the request to be cancelled which sets the RequestContext.cancelTk at the server end.
• Split EditorApi into a serializable EditorRpicApi and a (non-serializable) EditorApi wrapper for use in the infoview. This is necessary because AbortSignal cannot simply be sent over to the editor.
• Clean up handling of Disposable subscriptions in infoview.ts a bit.
• Fix a racy test.

This spun out of leanprover-community/ProofWidgets4#141 which hand-rolls support for cancelling widget requests. However, it is challenging to ensure that computations are cancelled when the infoview closes, for example, without changing the extension. I also think it is better to use builtin mechanisms already present in RequestM than to implement our own.