Add macOS support

[cbenhagen]

Creating a new sub issue for #51 as this is a pretty easy task as soon as flutter/flutter#65222 is closed.

/cc @hacker1024

[chongbo2013]

is support macos? thank u

[nmfisher]

This would be a welcome feature - if anyone wants to contribute, please submit a PR.

[chipweinberger]

I added a comment to flutter/flutter#65222. I think it would be pretty simple to have macOS support if we require the user to put their isolate function in their main.dart file

[chipweinberger]

PR: #153