`SimpleAssign`s target should be `StaticAssignable`, not `Node`

[faultyserver]

I'm pretty sure that SimpleAssign is only created when the left-hand side is a StaticAssignable (e.g., Var, Const, or Underscore). If the target is a pattern, then it becomes a MatchAssign, and if it is some method chain (e.g. x.y = 2), then it is converted into a Call to the y= method with x as the receiver.

With that, I think that the target property of SimpleAssign nodes can be restricted from Node to StaticAssignable. That guarantee is helpful when doing semantic analysis (i.e., myst-typecheck :) and avoids the need to downcast it whenever it is referenced.