Add .ixx as a possible C++ extension

This extension is used for a C++ module interface file, required for it to be automatically compiled as a module in Visual Studio (as seen [here](https://docs.microsoft.com/en-us/cpp/cpp/modules-cpp?view=msvc-160#basic-example)). Therefore, files with this extension should be highlighted as C++.

extensions/cpp/package.json

@@ -41,6 +41,7 @@
           ".ino",
           ".inl",
           ".ipp",
+          ".ixx",
           ".hpp.in",
           ".h.in"
         ],