Migrated to package:web

Migrated audio_service_web to use package:web instead of dart:html, dart:js & dart:js_util.

I also updated the example app and tested with it to see if the new web changes were working.

Pre-launch Checklist

[x] I read the [CONTRIBUTING.md] and followed the process outlined there for submitting PRs.
[x] My change is not breaking and lands in minor branch OR my change is breaking and lands in major branch.
[x] If I'm the first to contribute to the next version, I incremented the version number in pubspec.yaml according to the [pub versioning philosophy].
[x] I updated CHANGELOG.md to add a description of the change (format: * DESCRIPTION OF YOUR CHANGE (@your-git-username)).
[ ] I updated/added relevant documentation (doc comments with ///).
[ ] I ran dart analyze.
[ ] I ran dart format.
[ ] I ran flutter test and all tests are passing.