toLeopard: Serialize initial rotation style

[towerofnix]

This PR makes toLeopard represent the initial rotation style (which is already represented in IR and loaded by fromSb3).

• Resolves Preserve rotation style when converting to Leopard #117.

Implementation notes:

• Although we convert to the same Sprite.RotationStyle.ALL_AROUND/etc value both in block and initial-state serialization, we're mapping from different values. (sb3 block inputs like "all around" or "don't rotate" in the former, sb-edit values like "normal" or "none" in the latter.) Accordingly, it wasn't appropriate to introduce a constant mapping reused by both — but it's definitely a bit weird that sb-edit represent the input for "set rotation style" with the same values it uses for a sprite's initial rotation style.
• I had to change the way we stringify sprites to support including an arbitrary expression in the initial-state object. The important part is that each key maps to an already "stringified" value, i.e. the exact expression that will be inserted, instead of implicitly stringifying all values with JSON.stringify on the whole object.
• The output index.js file needs to import Sprite from Leopard's library file now, as well as Project (as it did before).

Tested with this project: https://scratch.mit.edu/projects/877138925/

And a "boring" project that makes sure the "set rotation style" blocks are still working.

Requested both @PullJosh and @adroitwhiz for review but an OK would be fine from either of you, this isn't a big pull request and improves Scratch compatibility for Leopard in a user-visible way!

[PullJosh]

Looks great! Thank you! 😄