UndeclaredVariable should re-lookup on reflective read/write

[MarcusDenker]

UndeclaredVariable now raises an exception when read or written. See for read:

runtimeUndeclaredReadInContext: aContext

	<debuggerCompleteToSender>
	^ UndeclaredVariableRead new
		messageText: 'Attempt to read undeclared variable ' , self name;
		variable: self;
		signal

We should, before raising the exception, re-lookup the name and if we find a Variable, delegate the access.

This will allow us in a second step to simplify Undeclared repair as we can just rely on re-compilation to fix installed methods (see #16664), for methods on the stack, the Undeclared re-lookup will make sure that the read and write will work for the case the name has been defined in the meantime.