use an assets folder for the app's fonts

[devoncarew]

• follow up to fix use of fixed width fonts in the console #2980 - list the app's fonts in an assets folder

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[parlough]

I've tried another approach in #2982 that moves the frame's prepared assets to its own directory, as it seems the root of the problem was its AssetManifest, which overrided the one generated for DartPad proper.

[devoncarew]

Closing in favor of #2982.