fix: Adjust fail method and ActionFailure type

[dummdidumm]

• add ActionFailure interface and use that publicly instead of the class. Prevents the false impression that you could do "instanceof" on the return type, fixes instanceof ActionFailure refers to the wrong object. #10361
• add uniqueSymbol to ActionFailure instance so we can distinguish it from a regular return with the same shape
• fix setup to actually run test/types

Someone with a non-windows machine has to check in the type diff (and double check it) because I can't do it properly. There's a bug in dts-buddy somewhere that break path resolving on Windows (I'm guessing a missing \ to / conversion).

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[Rich-Harris]

Checked in the types — LGTM, but double-check me...