Number formatt method to add thousand separator to number strings.

format_number.gd

func format_number(number: Variant, thousand_separator: String = ",") -> String:
	var number_str: String = str(number)
	var start_index: int = number_str.find(".")
	
	if start_index == -1:
		start_index = number_str.length()
	
	for i in range(start_index - 3, 0, -3):
		number_str = number_str.insert(i, thousand_separator)
	
	return number_str