Add custom editor for sprites to allow setting the origin

[adam-coster]

For our current pipeline, the only thing we need GameMaker for is to set the origin of sprites. If we pull this functionality into Stitch for VSCode, then we can mostly avoid GameMaker completely in the art pipeline.

This would require a custom webview/editor for sprite yy files, with the following features:

• Dropdown for choosing an origin method (must match GameMaker's)
• A clickable rendering of the initial sprite frame (or all frames?), where clicking would set the origin (and change the method to "manual")