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/native/src/node_pty/unix/pty.h

@@ -0,0 +1,31 @@
+// Copyright (c) 2021, Leroy Hopson (MIT License).
+
+#ifndef GODOT_XTERM_PTY_H
+#define GODOT_XTERM_PTY_H
+
+#include <FuncRef.hpp>
+#include <Godot.hpp>
+
+namespace godot {
+
+class PTYUnix : public Reference {
+  GODOT_CLASS(PTYUnix, Reference)
+
+public:
+  Array fork(String file,
+             int _ignored, /* FIXME: For some reason Pipe throws
+                       ENOTSOCK in read callback if args (or another non-empty,
+                       non-zero) value is in this position. */
+             PoolStringArray args, PoolStringArray env, String cwd, int cols,
+             int rows, int uid, int gid, bool utf8, Ref<FuncRef> on_exit);
+  Array open(int cols, int rows);
+  godot_error resize(int fd, int cols, int rows);
+  String process(int fd, String tty);
+
+  void _init();
+  static void _register_methods();
+};
+
+} // namespace godot
+
+#endif // GODOT_XTERM_PTY_H