Update to modern Dart style and lints

[eseidel]

Added package:lints/recommended and fixed all the lint warnings.

API BREAK: EventSourceReadyState values are now camelCase instead of SCREAMING_CAPS.

This was almost entirely automated. I just pulled down the package, added the lints dependency and let the quickfixes in visual studio code do their thing.

I ran dart test all tests passed.

https://github.com/dart-lang/http/issues/1 is now resolved, presumably removing the need for a separate BrowserClient, but I did not attempt to change that part of the API.

[eseidel]

The author line from the pubspec.yaml was only removed because pub yelled at me about it. It's no longer used I guess. 🤷