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"))

examples/terminal/terminal.tscn

@@ -1,20 +1,21 @@
 [gd_scene load_steps=3 format=2]
 
-[ext_resource path="res://addons/godot_xterm/nodes/terminal/terminal.gd" type="Script" id=1]
-[ext_resource path="res://addons/godot_xterm/nodes/pseudoterminal/pseudoterminal.gdns" type="Script" id=2]
+[ext_resource path="res://addons/godot_xterm/nodes/pty/unix/pty_unix.gd" type="Script" id=2]
+[ext_resource path="res://examples/terminal/terminal.gd" type="Script" id=3]
 
 [node name="Terminal" type="Control"]
 anchor_right = 1.0
 anchor_bottom = 1.0
-focus_mode = 1
-script = ExtResource( 1 )
+focus_mode = 2
+script = ExtResource( 3 )
 __meta__ = {
 "_edit_use_anchors_": false
 }
 
-[node name="Pseudoterminal" type="Node" parent="."]
+[node name="PTY" type="Node" parent="."]
 script = ExtResource( 2 )
-
-[connection signal="data_sent" from="." to="Pseudoterminal" method="write"]
-[connection signal="size_changed" from="." to="Pseudoterminal" method="resize"]
-[connection signal="data_sent" from="Pseudoterminal" to="." method="write"]
+terminal_path = NodePath("..")
+env = {
+"COLORTERM": "truecolor",
+"TERM": "xterm-256color"
+}