Add player_state dictionary to manage player state names for FSM demo

[Yaxian]

Add player_state to manage player state.

static var player_state := {
	"previous": "previous",
	"jump": "jump",
	"idle": "idle",
	"move": "move",
	"stagger": "stagger",
	"attack": "attack",
	"die": "die",
	"dead": "dead",
	"walk": "walk",
}

I tried to use enum, like

state.gd

enum BaseState {
 	PREVIOUS
}

player_state.gd

extends "res://state_machine/state.gd"

enum PlayerState {
	JUMP
}

But the function signature verification fails. Is there a good way to manage state in state machine, like type check, code hints?