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/LICENSE.md

@@ -1,5 +1,7 @@
 The code under the `node_pty` directory is taken from the [node-pty project](https://github.com/microsoft/node-pty), which in turn contains code from the [Tmux project](http://tmux.sourceforge.net/).
 
+The code has been modified to remove references to node/V8 and make it compatible with GDNative. Any copyrightable modifications are released under the [same license](/addons/godot_xterm/LICENSE) as the rest of the GodotXterm project.
+
 ### Node-pty License
 
 ```
@@ -92,4 +94,4 @@ ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
 IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
-```
+```