Should figure out how to check sync/async types for decorators

In the example below, the last function, gas, should be flagged, since a synchronous function can't depend on an async event. Currently, our typing does not flag it. (I don't know that it's actually possible with Python.)

def fs() -> int:
    return 1

async def fa() -> int:
    return 1

# sync-sync - OK
@event(fs)
def gss() -> int:
    return 3

# async-async - OK
@event(fa)
async def gaa() -> int:
    return 3

# sync-async - OK
@event(fs)
async def gsa() -> int:
    return 3

# async-sync - should be flagged
@event(fa)
def gas() -> int:
    return 3

Start by investigating the typing behavior of the event decorator using the four sync/async examples in the issue. Done means determining whether the type system can flag gas, where a synchronous function depends on an async event, while accepting the other three combinations.