GdScript serial port comms

Based on https://youtu.be/nOKno82_gd0?si=i7zCyCndsfXXC8QI

With the following additions:

• Use an Autoload in gdscript to call the C# SerialPort functions (allowing you to keep using GdScript as your main language).
• Add the toggle mono build plugin so that the C# build can be turned off (since we're only using one file that won't change); shaves some time off each run.

Prerequisites

• .NET version of Godot 4
• .NET runtime installed

Setup (summarized from video):

1. Make sure there is a your-project-name.csproj file (should be auto created when you make a C# script)
2. Install the System.IO.Ports .NET package: dotnet add package System.IO.Ports --version 7.0.0
3. Build the project at least once
4. Activate the Mono Build Toggler plugin
5. Specify your port name and baud rate in serial_port_handler.gd