`reactive.Effect` should warn if function returns a value

For example, if a user writes this:

@reactive.Effect
def foo():
  return 123

It will run and the user will not receive any indication that the code is probably not doing what they expect. (It will be flagged by a static type checker like pyright, but many users don't use them.)

This should raise an informative warning. (And come to think of it, we should do that in R as well. But now that I think about it again, we can't tell in R if a function just returned a value because it happened to be the last expression when running the function, or if the function was really intended to return the value, so we can't add this check in R.)

Start at the Python reactive.Effect entry point and trace how decorated functions are run. Determine where a returned value can be detected and how warnings are emitted. Done means an Effect function returning a value produces an informative warning, while functions without a return value continue to run normally.