[code-watcher] Add CodeWatcherMode option.

There are three option values:
- Disabled: module does nothing (wow).
- Remove: module removes matching messages and sends temporary message
          to instruct user what to do.
- Upload: module uploads detected code to configured Haste Server.

modules/CodeWatcher/StringExtension.cs

@@ -5,13 +5,10 @@ public static class StringExtension
     // https://stackoverflow.com/a/23408020
     public static IEnumerable<string> SplitToLines(this string input)
     {
-        using (var reader = new StringReader(input))
+        using var reader = new StringReader(input);
+        while (reader.ReadLine() is { } line)
         {
-            string line;
-            while ((line = reader.ReadLine()) != null)
-            {
-                yield return line;
-            }
+            yield return line;
         }
     }
 }