Feature: 2D Space Shooter UI Pass using UI Toolkit (#55)

* First pass at making UI with UIToolkit

* UI Pass

* Final pass on UI for 2D Space Shooter--new buttons, new sanitized input fields, new in game UI for player and power ups, and an overall logic pass to hook everything up

* Quick fix to make the other players' UI follow their ships

* Added a sorting group component to the ships so that there's no weird clipping overlap between two ships

* Iteration of visuals of the UI, going for more of a scifi look and reusing assets to increase cohesion

Author: jilfranco-unity