Add FileAccess.store_string_as_file()

[timothyqiu]

Currently we have the static method get_file_as_string() to easily get file content as a String.

There are also needs to save one String as a text file, e.g., saving the return value of JSON.stringify() as a JSON file. So I suggest adding a corresponding static method.

func store_string_as_file(text: String, path: String) -> Error