Add RMain::wait_initialized()

Author: lionel-

crates/ark/src/interface.rs

@@ -490,6 +490,17 @@ impl RMain {
         R_MAIN_INIT.get().is_some()
     }
 
+    /// Wait for `RMain` singleton initialization
+    ///
+    /// Note that R might still not have finished starting up.
+    /// See `wait_r_initialized()`.
+    ///
+    /// Thread-safe. But note you can only get access to the singleton on the R
+    /// thread.
+    pub fn wait_initialized() {
+        R_MAIN_INIT.wait();
+    }
+
     /// Access a reference to the singleton instance of this struct
     ///
     /// SAFETY: Accesses must occur after `start_r()` initializes it, and must