Deprecate $ExpectError in favour of @ts‑expect‑error

[ExE-Boss]

TypeScript 3.9 added the @ts‑expect‑error compiler directive (microsoft/vscode#92093), which makes dtslint’s $ExpectError obsolete.

dtslint shouldn’t consider lines with @ts‑expect‑error instead of $ExpectError as failing the build in TypeScript 3.8 or older and should prefer it for newer DefinitelyTyped projects.