Add an example app

[devoncarew]

From https://github.com/flutter/flutter/issues/136217, we should add an example app to package:process in order to improve the pub scores.

[IldySilva]

i guess it can be closed as completed @devoncarew

[devoncarew]

It doesn't look like there's an example for package:process (https://github.com/dart-lang/tools/tree/main/pkgs/process). It's not a pressing issue, but would be useful for completeness.