Add CW keying, function key messages and the documentation

The function keys send through cwdaemon or a WinKeyer, with N1MM's message
macros. Escape stops sending.

Settings are read with the reflection serializer rather than a generated one:
the generated one hands back null for every property the file leaves out
instead of the value the property is declared with, which crashed the program
the first time a new setting was added.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Nonemm.Keying/MessageSender.cs

@@ -0,0 +1,17 @@
+namespace Nonemm.Keying;
+
+/// Something that sends a message on the air: a CW keyer, or a voice keyer
+/// playing a recording.
+public interface MessageSender : IDisposable
+{
+    bool IsReady { get; }
+
+    /// Sends the text. Whatever is already going out is finished first unless
+    /// `Abort` is called.
+    Task SendAsync(string text, CancellationToken cancellation = default);
+
+    /// Stops sending straight away, which is what Escape does mid-message.
+    Task AbortAsync(CancellationToken cancellation = default);
+
+    Task SetSpeedAsync(int wordsPerMinute, CancellationToken cancellation = default);
+}