Consider new Id(...) mechanism to retrieve the id of a resource

This would work similar to `Canonical(...)` but would return the `id` instead (obviously).

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/example.20FSH.20Bundle.20transaction.20with.20Create.20entries/near/230901491

Start by reviewing how Canonical(...) is implemented and read the linked FHIR Shorthand discussion about transaction examples. Define the expected Id(...) behavior so it returns a resource id, then verify the implementation with the relevant existing Canonical(...) behavior and tests.

Written by the indexing model from the issue text.