[NEOS-1327] Add check to JS validation that there is a return statement

[nickzelei]

We should check the user's code to ensure they are returning something.

We could start by doing a string check on the last line they have defined to see if there is a return keyword present. It might be more complicated than that, not sure.

Add integration test for this.

_{From SyncLinear.com | NEOS-1327}