Implement pcRange on Context using #sourceNodeExecuted

[MarcusDenker]

We have right now #pcRangeContextIsActive: on Context. This means that the highlight of the debugger depends on the UI to tell if the context is the top most context or not.

But we have already #sourceNodeExecuted, which returns exactly the right AST node.

We thus could implement pcRange on Context as:

^self sourceNodeExecuted debugHighlightRange

and remove quite some code that is not needed anymore