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/extension/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "helgobox-extension"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+reaper-fluent.workspace = true
+reaper-medium.workspace = true
+reaper-low.workspace = true
+reaper-macros.workspace = true
+anyhow.workspace = true
+libloading.workspace = true
+
+[lib]
+name = "reaper_helgobox"
+crate-type = ["cdylib"]
+
+[lints.clippy]
+enum_glob_use = "deny"