Preserve block comment positions when (de)serializing

[gonfunko]

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Problem

Currently, the location of block comments in the workspace are not preserved/restored when (de)serializing them, unlike workspace comments.

Request

To do this, we should:

• Extend ICommentIcon and its implementation with setBubbleLocation(location: Coordinate): void and getBubbleLocation(): Coordinate methods.
• Update blockToDom() to call getBubbleLocation() and persist the coordinate under x and y attributes in the block comment XML, and update applyCommentTagNodes() to move the comment to the specified location when deserializing. I realize we're not adding new functionality to XML serialization in general, but this particular change makes work on upgrading Scratch easier and is quite small/contained.
• Update saveState() and loadState() in CommentIcon to persist and restore the comment coordinates when using JSON serialization.
• Update the tests to exercise moving the comment and (de)serialization

Alternatives considered

No response

Additional context

No response

[BeksOmega]

This LGTM!

Extending ICommentIcon is a breaking change though, so we need to create a V12 branch and merge this into that. If you want to do that, just create an rc/v12.0.0 branch off of develop and push it to upstream (instead of your fork)