Feature: Add pragma to have a type check failure return `Mixed` data type.

[Raynos]

Having an opt-in way to have a type checker failure return Mixed would allow us to find cascading failures due to line A failing causing line B to fail.

This on it's own would just be very noisy but can be combined with the notion of "only check one function" or "only check these lines that have changed". This allows us to type check the entire file and only report errors related to certain lines.