Align `:method` Expr return value between interpreter and codegen (#58076)
Interpreter/inference think the 3-argument `:method` Expr returns
`nothing`. Codegen thinks it returns the new method. I think the latter
makes more sense, because it lets us write explicit syntax-level
dependency links between method definitions and constants (used e.g. for
external abstract interpreters), which is something that Revise may need
in the future. Adjust the interpreter/inference to properly return the
method.