Send Cancellation Token to TypeScript Language Server

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Cancellation token is issued when the next operation happens. For example, when typing abc, Vetur will query TS Server for diagnostics on a, ab, abc. If typing of abc finishes before a's diagnostic request is responded, vscode-languageclient should send a cancellation token.

In this case, Vetur should forward that cancellation token to TS Server so it can stop computing outdated diagnostics/auto-completion. This should reduce a lot of CPU/memory usage from VLS.