Inconsitent use of HI inside GamepadPlugin

[DavidVollmers]

Hello,

https://github.com/liftoff/HumanInput/blob/b96967b41afb4c7293848be19a4061024132bdbc/src/gamepad.js#L241

https://github.com/liftoff/HumanInput/blob/b96967b41afb4c7293848be19a4061024132bdbc/src/gamepad.js#L247

HI is used instead of this.HI which results in an error when managing the HI instance by yourself and when no global HI is defined...

A hacky hotfix for this issue can be:

eval( 'window.HI = { scope: "" }' )