[red-knot] support del statement and deletion of except handler names

[carljm]

At runtime, names bound in an except handler are del ed at the end of the except handler block. We should model this as if there were a del statement for the name at the end of the block. And we should also support real del statements.

In both cases, it should take the form of a Definition that causes the name to be unbound.

[AlexWaygood]

Oops, accidentally closed this. It's not fixed!