Convert to "new" Unity input system

[lukasgolson]

• It will make controls very easy to rebind at runtime
• Makes button layouts easier (i.e., different schemes for flying planes vs on foot)
• Make working with input much more efficient in the future, and would allow us to add support for nearly any input device within minutes.

[GartzenDeHaes]

I've been working on this for local split screen support. FYI, the new input system does not seem to work with IMGUI.

"The Input System cannot generate input for IMGUI." https://docs.unity.cn/Packages/com.unity.inputsystem@1.2/manual/KnownLimitations.html

Update: works in desktop, but can't click anything on the mobile simulator.