[ffigen] Implement YAML -> tool/ffigen.dart migrator

[jakobkordez]

Description

Implements a migrator from YAML config to tool/ffigen.dart

Related Issues

Resolves #2804

PR Checklist

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've run dart tool/ci.dart --all locally and resolved all issues identified. This ensures the PR is formatted, has no lint errors, and ran all code generators. This applies to the packages part of the toplevel pubspec.yaml workspace.
• All existing and new tests are passing. I added new tests to check the change I am making.
• The PR is actually solving the issue. PRs that don't solve the issue will be closed. Please be respectful of the maintainers' time. If it's not clear what the issue is, feel free to ask questions on the GitHub issue before submitting a PR.
• I have updated CHANGELOG.md for the relevant packages. (Not needed for small changes such as doc typos).
• I have updated the pubspec package version if necessary.

[liamappelbe]

Thanks for the PR @jakobkordez. The Dart API is still in flux though, so I don't think we can add the migration tool yet. I have a bunch of API cleanup I want to do first.