expression-level type coverage measurement

[carljm]

Description

Real-world type-checker users often want to get a sense of how well-covered their code is with static types. One granular way to measure this is "percentage of expressions typed with a dynamic type".

We would also have an internal use case for measuring "percentage of expressions typed with a Todo type" as a measure of red-knot progress.

[janosh]

this would be a great feature! maybe some inspiration to be taken from https://github.com/plantain-00/type-coverage