Support compilation to webassembly (should be fairly easy!)

[willsmanley]

This library currently uses package:js/js.dart and package:js/js_util.dart, which are not compatible with WebAssembly (Wasm) builds. This results in build errors when targeting Wasm.

It would be great to migrate to dart:js_interop for compatibility with Wasm. This change would help developers who are targeting Wasm with their Flutter web applications (faster and better!)

The change should not be very difficult (i will look at it if i have time) but it would make a huge difference

[willsmanley]

Looks like i was able to do it fairly quickly: https://github.com/willsmanley/mixpanel-flutter

I'll open a PR now

[willsmanley]

looks like we already have an approved PR, i didnt see until now: #167