Allow embedded Snacks to view code other than the `App.js` file

We'd need a property to set the file to view in an embedded Snack. We need this for Skia, to open `Breathe.tsx` instead of `App.tsx`. With that, users can edit the breathe logic to preview it, while still keeping the web-wrapping logic in the entrypoint.

Start by locating the embedded Snack entrypoint and the current logic that opens App.js or App.tsx. Read how the embedded file is selected, then compare the requested Breathe.tsx use case. Done means an embedded Snack can specify another file for viewing while retaining its entrypoint for the web-wrapping logic.

Written by the indexing model from the issue text.