Warning/Error for unassigned expressions/return values

Feature

Adding a warning or error if an expression is not assigned would encourage callers to handle return types and pick up on changes to APIs.

Pitch

Imagine you have this api:

def fleeb_the_foo() -> None:
    print("fleebing")

def main() -> None:
    fleeb_the_foo()

Then you change it later:

def fleeb_the_foo() -> Bool:
    print("I failed to fleeb!")
    return False

def main() -> None:
    fleeb_the_foo() # <<--- a warning or error here!

If you don't want to handle the warning/error:

def fleeb_the_foo() -> Bool:
    print("I failed to fleeb!")
    return False

def main() -> None:
    _ = fleeb_the_foo() # error is suppressed

This is already implemented to some degree with pylint for unassigned expressions with [expression-not-assigned]

str(42) == "42"  # [expression-not-assigned]

Start by comparing the proposed behavior with pylint's expression-not-assigned check, which is linked in the issue. Clarify which unassigned return values should produce diagnostics and how explicit _ = suppression should work. Done means the behavior and diagnostic rules are defined well enough to implement and test.