feat: Support for iOS

[markvideon]

Addressing #19 , iOS support is based on gamepads_darwin with minor adjustments - e.g. the platform checks refer to relevant versions of iOS.

Note that the example project as-is will fail due to the published release of gamepads 0.1.1 not depending on gamepads_ios. Manually adding gamepads_ios: ^0.1.1 as a dependency to the example project will allow you to test everything links as expected.