Implement LSP contract for Semantic Tokens

[mattmasson]

Attempt to implement semantic token parsing using the LSP API - #217

Before:

After:

However, I'm not sure if it is working as expected as the "Developer: Inspect Editor Tokens and Scope" only seems to show semantic token info for the as keyword.

test script:

section foo;

myfunc = (table as table, param as record, optional hello) =>
    let
        a = table,
        b = hello
    in
        a;

This PR builds on #222 (which should get merged first).

[JordanBoltonMN]

Use this signature instead:
export function isCancellationError(value: unknown): value is PQP.CommonError.CancellationError

Also, would it be useful to add a isCancellationError to PQP?