Move models, redaction and scenarios into the core #39

Part of the core extraction (#35). Move what is already framework-independent into the core: models and types, redaction, user timings and the scenario class.

Scope

• Move them with their tests; the Angular package imports and re-exports them.
• Type tests for the core's public types.

Done when

• api:check and the type tests from the safety-net issue pass unchanged: the Angular package's public API is the same.