Type-level disparity between keyboard and gamepad input APIs

## What problem does this solve or what need does it fill?

Keyboard inputs are accessed with ` Res>`, and their associated variant enum is `KeyCode`.

Gamepad inputs are accessed with ` Res>`, and their associated variant enum is `GamePadButtonType`.

This makes writing code which abstracts over both keyboards and gamepads (such as input mapping or UI navigation) frustrating.

## What solution would you like?

Create a `GamepadButtonType` input resource, which assumes a single controller.

## What alternative(s) have you considered?

Merge `GamepadButton` and `GamepadButtonType`, likely with the former's name and latter's functionality.

Work around this in end-user code, likely with associated types or additional optional generics.

Make keyboard input behave the same way as gamepad input, supporting multiple input devices.

## Additional context

The fundamental reason for this divergence in API is that the gamepad version stores both which gamepad a button is coming from, as well as its button type. The keyboard API by contrast assumes a single input device.