Loosen dependencies on string_scanner (#4412)
Loosen dependencies on string_scanner (#4412)

Now that there's a new string_scanner in town, we're having dependency
resolution conflict because of flutter_markdown's tight dependency. This
patch loosens the dependency and resolves the conflict.
flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages