Refactor `Block` so that `output` is always a `str` or `None`

[AlexWaygood]

Block.output is incredibly un-type-safe at the moment:

cpython/Tools/clinic/clinic.py

Line 1710 in 49f238e

output: Any = None # TODO: Very dynamic; probably untypeable in its current form?

Most of the time, it's a str, but whenever we're inside one of the methods called by DSLParser.parse(), it's a list of strings. See my essay in python#106519 (comment) for more.