Split sandboxed reviewer iframe

[andrewsanchez]

Source split from ankitects#4289.

Breakdown sections covered:

• 3. Sandboxed inner reviewer iframe
• 4. Parent/iframe message passing
• 21. Typed answer replacement
• 22. MathJax support and font CORS handling
• 23. Image occlusion support
• 25. localStorage persistence shim
• 37. Backwards-compatibility shims

Source fidelity:

• This PR copies exact final file contents from PR Feat/Svelte port for reviewer ankitects/anki#4289.
• No implementation changes were made while splitting.
• Commit author is preserved as Luc Mcgrady.

Follow-up notes from the breakdown:

• postMessage currently copies card contents; this should be revisited for large cards.
• The localStorage shim needs more compatibility and size testing.
• Typed answer parsing may differ slightly for some nc: / cloze cases.