add gamestate, doc and fix

core/game_state/game_state.gd

@@ -0,0 +1,41 @@
+extends Node
+
+const SAVE_PATH: String = "user://savegame.json"
+
+var save_data: Dictionary = {}
+var is_loaded: bool = false
+
+func _ready() -> void:
+	load_game()
+
+func save_game() -> void:
+	var save_file = FileAccess.open(SAVE_PATH, FileAccess.WRITE)
+	var save_nodes = get_tree().get_nodes_in_group("Saveable")
+	var new_save_data: Dictionary = {}
+	
+	for node in save_nodes:
+		if not node.has_method("get_save_data"):
+			continue
+		
+		var node_data = node.get_save_data()
+		var node_path = str(node.get_path())
+		new_save_data[node_path] = node_data
+
+	var json_string = JSON.stringify(new_save_data)
+	save_file.store_line(json_string)
+
+func load_game() -> void:
+	if not FileAccess.file_exists(SAVE_PATH):
+		is_loaded = true
+		return
+		
+	var file = FileAccess.open(SAVE_PATH, FileAccess.READ)
+	var json_string = file.get_as_text()
+	
+	var json = JSON.new()
+	var parse_result = json.parse(json_string)
+	
+	if parse_result == OK:
+		save_data = json.data
+	
+	is_loaded = true

core/game_state/game_state.gd.uid

@@ -0,0 +1 @@
+uid://b17g2w2g101o6