This PR modifies the web build slightly to create an `AssetManifest.json`, that is a JSON(base64)-encoded version of the `AssetManifest.bin` file.

_(This should enable all browsers to download the file without any interference, and all servers to serve it with the correct headers.)_

It also modifies Flutter's `AssetManifest` class so it loads and uses said file `if (kIsWeb)`.

### Issues

* Fixes https://github.com/flutter/flutter/issues/124883

### Tests

* Unit tests added.
* Some tests that run on the Web needed to be informed of the new filename, but their behavior didn't have to change (binary contents are the same across all platforms).
* I've deployed a test app, so users affected by the BIN issue may take a look at the PR in action:
  * https://dit-tests.web.app