fix gamelab debugger line numbers by accounting for hidden sharedBlocks and helperLibraries

[cpirich]

• When various commits added "invisible" code above the user's code in Gamelab (validation code, helper libraries, shared blocks), they broke the debugger's calculation of line numbers for highlighting code and enforcing breakpoints. Fortunately, we have a feature within the interpeter/debugger for handling this case (called userCodeStartOffset), so we just need to wire it up in gamelab so we can get the desired effect.