Use a state machine for the player scene

[GTcreyon]

Describe the feature you'd like
Rewrite player.gd and the player scene in general to make use of a state machine instead of a single monolithic script.

Do you intend to contribute this yourself? If so, how do you plan on doing it?
I can provide a state machine implementation designed with platforming in mind, derivative of https://github.com/Charpurrr/SuperMarioSolarEngine. I may be able to code the states themselves, but perhaps not.

[Koopa1018]

Would facilitate solving of #158 as well.