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/terminal/terminal.gd

@@ -0,0 +1,7 @@
+extends "res://addons/godot_xterm/nodes/terminal/terminal.gd"
+
+onready var pty = $PTY
+
+
+func _ready():
+	pty.fork(OS.get_environment("SHELL"))