Warn on (accidentally) unreachable code blocks

[AlexWaygood]

Mypy and pyright both offer opt-in lints which, when enabled, cause them to complain about the else branch in this snippet:

def f(x: int | str):
    if isinstance(x, int):
        print("It's an int")
    elif isinstance(x, str):
        print("It's a string")
    else:
        print("It's something else")

They can both detect that the else branch is unreachable, and that this is probably an accident, indicating a logic error somewhere in the function. Mypy's version can be opted into using --warn-unreachable on the command line; pyright's version requires reportUnreachable=true.

For feature parity with these tools, we should aim to provide a similar lint.

Note that the lint should only complain about accidentally unreachable code. There's a lot of code that ty infers as being unreachable that is nonetheless deliberately unreachable, which we should not complain about:

• Any unreachable branches that consist solely of a raise statement should not be reported
• Any unreachable branches that consist solely of an assert_never() call should not be reported
• Any branches that are unreachable due to a sys.version_info, sys.platform, or if TYPE_CHECKING check should not be reported