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.cpp

@@ -0,0 +1,108 @@
+// Copyright (c) 2021, Leroy Hopson (MIT License).
+
+#include "pipe.h"
+#include "libuv_utils.h"
+#include <Dictionary.hpp>
+#include <InputEventKey.hpp>
+#include <OS.hpp>
+#include <ResourceLoader.hpp>
+#include <Theme.hpp>
+#include <Timer.hpp>
+#include <algorithm>
+#include <thread>
+#include <vector>
+#include <xkbcommon/xkbcommon-keysyms.h>
+
+using namespace godot;
+
+void Pipe::_register_methods() {
+  register_method("_init", &Pipe::_init);
+
+  register_method("poll", &Pipe::_poll_connection);
+  register_method("open", &Pipe::open);
+  register_method("write", &Pipe::write);
+
+  register_signal<Pipe>("data_received", "data",
+                        GODOT_VARIANT_TYPE_POOL_BYTE_ARRAY);
+}
+
+Pipe::Pipe() {}
+Pipe::~Pipe() {}
+
+void Pipe::_init() {}
+
+void _poll_connection();
+
+void _read_cb(uv_stream_t *handle, ssize_t nread, const uv_buf_t *buf);
+
+void _write_cb(uv_write_t *req, int status);
+
+void _alloc_buffer(uv_handle_t *handle, size_t suggested_size, uv_buf_t *buf);
+
+godot_error Pipe::open(int fd, bool ipc = false) {
+  RETURN_IF_UV_ERR(uv_pipe_init(uv_default_loop(), &handle, ipc));
+
+  handle.data = this;
+
+  RETURN_IF_UV_ERR(uv_pipe_open(&handle, fd));
+  RETURN_IF_UV_ERR(uv_stream_set_blocking((uv_stream_t *)&handle, false));
+  RETURN_IF_UV_ERR(
+      uv_read_start((uv_stream_t *)&handle, _alloc_buffer, _read_cb));
+
+  return GODOT_OK;
+}
+
+godot_error Pipe::write(String p_data) {
+  char *s = p_data.alloc_c_string();
+  size_t len = strlen(s);
+
+  uv_buf_t bufs[] = {{.base = s, .len = len}};
+
+  uv_write_t req;
+
+  req.data = s;
+
+  uv_write(&req, (uv_stream_t *)&handle, bufs, 1, _write_cb);
+
+  uv_run(uv_default_loop(), UV_RUN_NOWAIT);
+
+  return GODOT_OK;
+}
+
+int Pipe::get_status() {
+  _poll_connection();
+  return status;
+}
+
+void Pipe::_poll_connection() { uv_run(uv_default_loop(), UV_RUN_NOWAIT); }
+
+void _read_cb(uv_stream_t *handle, ssize_t nread, const uv_buf_t *buf) {
+  Pipe *pipe = static_cast<Pipe *>(handle->data);
+  if (nread < 0) {
+    switch (nread) {
+    case UV_EOF:
+      // Normal after shell exits.
+      return;
+    case UV_EIO:
+      // Can happen when the process exits.
+      // As long as PTY has caught it, we should be fine.
+      return;
+    default:
+      UV_ERR_PRINT(nread);
+    }
+    return;
+  }
+
+  PoolByteArray data;
+  data.resize(nread);
+  memcpy(data.write().ptr(), buf->base, nread);
+
+  pipe->emit_signal("data_received", data);
+}
+
+void _write_cb(uv_write_t *req, int status) {}
+
+void _alloc_buffer(uv_handle_t *handle, size_t suggested_size, uv_buf_t *buf) {
+  buf->base = (char *)malloc(suggested_size);
+  buf->len = suggested_size;
+}