AK: Add const version of Vector::first_matching

Author: https://github.com/kleinesfilmroellchen
Commit: https://github.com/SerenityOS/serenity/commit/7b145d810a
Pull-request: https://github.com/SerenityOS/serenity/pull/13516
Issue: https://github.com/SerenityOS/serenity/issues/65

AK/Vector.h

@@ -173,6 +173,17 @@ public:
         return {};
     }
 
+    template<typename TUnaryPredicate>
+    Optional<VisibleType const&> first_matching(TUnaryPredicate predicate) const requires(!contains_reference)
+    {
+        for (size_t i = 0; i < size(); ++i) {
+            if (predicate(at(i))) {
+                return Optional<VisibleType const&>(at(i));
+            }
+        }
+        return {};
+    }
+
     template<typename TUnaryPredicate>
     Optional<VisibleType&> last_matching(TUnaryPredicate predicate) requires(!contains_reference)
     {