Fix for multi-code-block statements and exit.

Fixes https://github.com/enigma-dev/enigma-dev/issues/911

Turns this:
```
//code block 1:
exit;

//Code block 2:
show_message("HI");
```

Into this:
```
  {
    goto block_end_0;;    
  }
  block_end_0:
  {
    show_message("HI");
  }
```

Note that these block labels are only created if "exit" is called at least once inside them. If the current block level couldn't be determined (which is common in scripts, timelines, etc.) then the previous behavior of "return 0" is used. This causes scripts, timelines, etc. to act at least as accurately as they used to.