Added TargetPlatform

[nilsreichardt]

Sometimes you need to show different widgets for the platforms. To achieve this, you can use Theme.of(context).platform == TargetPlatform.isAndroid. But this is a little bit long. So I added this methods:

context.platform => TragetPlatform
context.isAndroid => bool
context.isIOS => bool
context.isWindows => bool
context.isMacOS => bool
context.isLinux => bool
context.isFuchsia => bool

Short questions: context.isIOS or context.isIOs? context.isMacOS or context.isMacOs?