Preserve block comment IDs when loading from saved file

[gonfunko]

Currently we initialize block comment IDs to a UUID, but in cases where a saved project is loaded this will cause problems with the VM, because the VM associates comments with their saved IDs, but we'll be sending comment events with the new random UUIDs. This may be more straightforward to address on the VM side by e.g. rewriting the comment IDs the VM stores in its lookup table to be $(BLOCK_ID)_comment or the like.

[cwillisf]

As long as project file compatibility works correctly in both directions -- new can load old projects AND old can load new projects -- I'm OK with changing the runtime format for these IDs. That "as long as" might be doing a lot of work there, though...? Let me know if it makes sense to work together on this one or meet to talk it through in detail.