Check validity of key code strings

^{_{When I'm done there will be no more stringly-typed methods left}}

There exists a simple mapping between the `KeyCode` enum and string representations used by the InputManager, [the pattern for which is listed in the Unity Manual](https://docs.unity3d.com/Manual/ConventionalGameInput.html). It would be great if the plugin could check for a valid string literal and provide a migration path to the more strongly typed overload which takes a `KeyCode` instead of a `string`.

- [`GetKey`](https://docs.unity3d.com/ScriptReference/Input.GetKey.html)
- [`GetKeyDown`](https://docs.unity3d.com/ScriptReference/Input.GetKeyDown.html)
- [`GetKeyUp`](https://docs.unity3d.com/ScriptReference/Input.GetKeyUp.html): Take a [string representing a key](https://docs.unity3d.com/Manual/ConventionalGameInput.html).

Sister Issue: #134 for virtual buttons/axis.