Improve Flattened type

[Jym77]

Part of #1037

• Add an alfa-act/Question.Metadata type that captures the expected shape of Metadata. Given that the shape is already enforced in the Oracle type, this ensures it at rule building time.
• Fix the inference of Input, Target, and Subject types in the Flattened export.
• Update TypeScript. We're currently locked out of 4.9 because of Expression produces a union type that is too complex to represent microsoft/TypeScript#53234 😞
• Add some type annotations in some import statements.
• Default empty question metadata to {} rather than never, this avoids never creeping in some weird places.
• Rules asking no questions now accept anything as oracle… since they'll never call it…