AK: Use AK_MAKE_DEFAULT_MOVABLE to avoid mistakes in default impls

Author: https://github.com/BenWiederhake
Commit: 041f5e931d
Pull-request: https://github.com/SerenityOS/serenity/pull/19422

AK/SourceGenerator.h

@@ -36,6 +36,8 @@ public:
     }
 
     SourceGenerator(SourceGenerator&&) = default;
+    // Move-assign is undefinable due to 'StringBuilder& m_builder;'
+    SourceGenerator& operator=(SourceGenerator&&) = delete;
 
     ErrorOr<SourceGenerator> fork()
     {