Rewrite inject_dartpad and expose some functionality in library

[parlough]

Previously web/inject_dartpad.dart was just written as a script for dart.dev and docs.flutter.dev, but the logic wasn't really reusable in other contexts and didn't have much error handling. This PR rewrites and moves the core logic into a new library and class at lib/inject_dartpad.dart that other clients can take advantage of through using this package as a git dependency.

Besides being a public library, the new library also makes a few improvements over the original script.

• Each embed independently handles its initialization and code update, resulting in shorter delays.
• Lazily loads embeds outside the current viewport.
• Replaces usage of jsify with directly constructing the appropriate JS object.
• Adds handling and helpful error messages for when the embed isn't successfully added to the DOM.

The script in web/inject_dartpad.dart was then updated to use the new library, with a few additional improvements:

• Concurrently initializes multiple embeds rather than one at a time.
• Gracefully falls back to showing the code if DartPad fails to load in each iframe.
• Reuses an embed's index compared to others on the page for its iframe ID, simplifying the set up logic.

\cc @craiglabenz Since I can't assign you as a reviewer. I guess you don't have write access to this repo.