Improve actions macros (#3292)

- `actions!` now uses `#[action]` on each struct to reduce duplication.
- The `#[action]` macro now works on unit structs.
- Renamed `menu::unused` to `menu::init` and added more explanation in
comments.

Release Notes:

- N/A

crates/zed2/src/main.rs

@@ -57,10 +57,7 @@ use zed2::{
 mod open_listener;
 
 fn main() {
-    //TODO!(figure out what the linker issues are here)
-    // https://github.com/rust-lang/rust/issues/47384
-    // https://github.com/mmastrac/rust-ctor/issues/280
-    menu::unused();
+    menu::init();
     let http = http::client();
     init_paths();
     init_logger();