Imports https://github.com/dnfield/flutter_svg into this repository,
with history, and updates it to follow repository conventions:

- Updates min SDKs.
- Removes analysis options and fixes resulting warnings.
- Autoformats.
- Updates pubspecs to follow repo standard.
- Updates repo tooling to allow the `flutter_svg_test` non-dev
  dependency on `flutter_test`.
- Adds repo metadata.
- Adds METADATA files.
- Bumps versions and slightly relax version constraints for
  vector_graphics* to allow the new versions.