feat: Add ArgGroup::is_required_set

This is a part of #4228

I thought I needed this for another change but it turned out I didn't.

src/builder/arg_group.rs

@@ -467,6 +467,12 @@ impl ArgGroup {
     pub fn get_id(&self) -> &Id {
         &self.id
     }
+
+    /// Reports whether [`ArgGroup::required`] is set
+    #[inline]
+    pub fn is_required_set(&self) -> bool {
+        self.required
+    }
 }
 
 impl From<&'_ ArgGroup> for ArgGroup {