AK: Add appendln helper to SourceGenerator

Author: https://github.com/Hendiadyoin1
Commit: https://github.com/SerenityOS/serenity/commit/6b20496758
Pull-request: https://github.com/SerenityOS/serenity/pull/12735
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

AK/SourceGenerator.h

@@ -77,6 +77,12 @@ public:
         }
     }
 
+    void appendln(StringView pattern)
+    {
+        append(pattern);
+        m_builder.append('\n');
+    }
+
 private:
     StringBuilder& m_builder;
     MappingType m_mapping;