feat: Added GamepadState that can be updated

[Levi-Lesches]

Usage:

final gamepads = await Gamepads.listGamepads();
final gamepad = gamepads.first;

// Now, at any point, you can check gamepad
print(gamepad.state.buttonInputs);
print(gamepad.state.analogInputs);

// When you're done with the gamepad, call dispose:
await gamepad.dispose();

Was not able to test on Windows due to some long compiler errors (see my message here) but will hopefully be able to test on Linux later this week

Fixes #12

[spydon]

The class looks good, but I bit strange that the user has to update it themselves?

[Levi-Lesches]

Updated the API, see the new example. I had to pass an instance of the plugin to the GamepadController constructor to automatically subscribe, but now you check state and call dispose

[spydon]

Other than some missing dartdocs and the formatting issue it looks good to me!

[Levi-Lesches]

Done, thanks for the quick review

[spydon]

Lgtm, thanks for the patience waiting for the final review, I was away for a conference.