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.

addons/godot_xterm/nodes/pty/libuv_utils.gd

@@ -0,0 +1,29 @@
+# Copyright (c) 2021, Leroy Hopson (MIT License)
+
+tool
+extends Object
+# Wrapper around libuv utility functions.
+# GDNative does not currently support registering static functions so we fake it.
+# Only the static functions of this class should be called.
+
+const LibuvUtils := preload("./libuv_utils.gdns")
+
+static func get_os_environ() -> Dictionary:
+	# While Godot has OS.get_environment(), I could see a way to get all environent
+	# variables, other than by OS.execute() which would require to much platform
+	# specific code. Easier to use libuv's utility function.
+	return LibuvUtils.new().get_os_environ()
+
+static func get_cwd() -> String:
+	# Use uv_cwd() rather than Directory.get_current_dir() because the latter
+	# defaults to res:// even if starting godot from a different directory.
+	return LibuvUtils.new().get_cwd()
+
+static func get_windows_build_number() -> int:
+	assert(OS.get_name() == "Windows", "This function is only supported on Windows.")
+	var release: String = LibuvUtils.new().get_os_release()
+	assert(false, "Not implemented.")
+	return 0
+
+static func new():
+	assert(false, "Abstract sealed (i.e. static) class should not be instantiated.")