Port `DirWatchSet` over to TS7

We said we would port https://github.com/microsoft/typescript-go/pull/4658 (issue https://github.com/microsoft/typescript-go/issues/4614) to 7.0.x following the move of the https://github.com/microsoft/typescript-go repo to https://github.com/microsoft/TypeScript. Well that's been done now, so we should cherry-pick e05860d9328a894189b49d9c43fb8a52e6c6d5a2 over to...some branch

Not sure exactly how this will work out - there is currently no `release-7.0` branch.