Add new PTY node (replaces Pseudoterminal node)

Uses fork of node-pty native code for forking pseudoterminals.
Uses libuv pipe handle to communicate with the child process.

- Paves the way for cross-platform (Linux, macOS and Windows) support.
- Renames Pseudoterminal to PTY (which is much easier to type and spell :D).
- Better performance than the old Pseudoterminal node. Especially when
  streaming large amounts of data such as running the `yes` command.
- Allows setting custom file, args, initial window size, cwd, env vars
  (including important ones such as TERM and COLORTERM) and uid/gid
  on Linux and macOS.
- Returns process exit code and terminating signal.

examples/menu/menu.gd

@@ -150,11 +150,7 @@ func _on_Terminal_key_pressed(data: String, event: InputEventKey) -> void:
 						"Terminal not Supported on Windows"
 					)
 				var scene = item.scene.instance()
-				var pty = (
-					scene
-					if OS.has_feature("JavaScript")
-					else scene.get_node("Pseudoterminal")
-				)
+				var pty = scene if OS.has_feature("JavaScript") else scene.get_node("PTY")
 				get_tree().get_root().add_child(scene)
 				visible = false
 				scene.grab_focus()