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/pipe.h

@@ -0,0 +1,64 @@
+// Copyright (c) 2021, Leroy Hopson (MIT License).
+
+#ifndef GODOT_XTERM_PIPE_H
+#define GODOT_XTERM_PIPE_H
+
+#include <Array.hpp>
+#include <FuncRef.hpp>
+#include <Godot.hpp>
+#include <Reference.hpp>
+#include <StreamPeer.hpp>
+#include <StreamPeerBuffer.hpp>
+#include <StreamPeerGDNative.hpp>
+#include <uv.h>
+
+namespace godot {
+
+class Pipe : public Reference {
+  GODOT_CLASS(Pipe, Reference)
+
+public:
+  uv_pipe_t handle;
+
+  static void _register_methods();
+
+  enum Status {
+    NONE,
+    CONNECTING,
+    CONNECTED,
+    ERROR,
+  };
+
+  int STATUS_NONE = Status::NONE;
+  int STATUS_CONNECTING = Status::CONNECTING;
+  int STATUS_CONNECTED = Status::CONNECTING;
+  int STATUS_ERROR = Status::ERROR;
+
+  Pipe();
+  ~Pipe();
+
+  void _init();
+
+  godot_error open(int fd, bool ipc);
+  int get_status();
+
+  godot_error write(String p_data);
+
+  void pause();
+  void resume();
+
+protected:
+  const godot_net_stream_peer *interface;
+
+public:
+  Status status;
+
+private:
+  void _poll_connection();
+
+  static godot_error _translate_error(int err);
+};
+
+} // namespace godot
+
+#endif // GODOT_XTERM_PIPE_H