Add issue templates, contribution guidelines, and code of conduct; update CI/CD workflows and project metadata (#3)

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>
Reviewed-on: #3

lib/list_or.dart

@@ -111,7 +111,7 @@ class ListOr<T> extends Object with ListMixin<T> {
   @override
   String toString() {
     if (_items.length == 1) {
-      return _items.firstOrNull.toString();
+      return _items.first.toString();
     }
     return _items.toString();
   }