Vendor helgoboss/helgobox (ReaLearn) as basis for custom UI fork

Stripped upstream git history; starting point for replacing the native
SWELL/Win32 mapping UI with something more suited to bulk editing.

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

plugin-reaper-realearn/main/lib/helgoboss-learn/src/source/mod.rs

@@ -0,0 +1,27 @@
+mod midi_source_value;
+pub use midi_source_value::*;
+
+mod midi_source;
+pub use midi_source::*;
+
+mod osc_source;
+pub use osc_source::*;
+
+mod raw_midi;
+pub use raw_midi::*;
+
+mod midi_source_script;
+pub use midi_source_script::*;
+
+mod feedback_script;
+pub use feedback_script::*;
+
+mod source_context;
+pub use source_context::*;
+
+mod color_util;
+
+#[cfg(test)]
+mod test_util;
+
+pub mod devices;