Add support for non-blocking IO mode to OS.execute_with_pipe.

[bruvzg]

Adds an extra argument to the OS.execute_with_pipe to allow pipes it creates to work in non-blocking mode.

e.g., a super primitive terminal emulation can be done without threads:

func _ready() -> void:
	if OS.get_name() == "Windows":
		pipe = OS.execute_with_pipe("cmd.exe", [], false)
	else:
		pipe = OS.execute_with_pipe("/bin/zsh", [], false)

func _process(delta: float) -> void:
	if pipe["stdio"].is_open():
		var buffer = PackedByteArray()
		while true:
			buffer.append_array(pipe["stdio"].get_buffer(2048))
			if pipe["stdio"].get_error() != OK:
				break;

		if !buffer.is_empty():
			$TextEdit.text += buffer.get_string_from_utf8()

func _on_line_edit_text_submitted(new_text: String) -> void:
	if pipe["stdio"].is_open() and !new_text.is_empty():
		pipe["stdio"].store_buffer((new_text + "\n").to_utf8_buffer())
		$LineEdit.text = ""

[akien-mga]

Thanks!