Add .ixx as a possible C++ extension
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++.
Microsoft/vscode,eamodio/vscode,microsoft/vscode,eamodio/vscode,microsoft/vscode,eamodio/vscode,microsoft/vscode,eamodio/vscode,microsoft/vscode,Microsoft/vscode,microsoft/vscode,Microsoft/vscode,eamodio/vscode,eamodio/vscode,eamodio/vscode,Microsoft/vscode,Microsoft/vscode,Microsoft/vscode,microsoft/vscode,microsoft/vscode,Microsoft/vscode,Microsoft/vscode,eamodio/vscode,Microsoft/vscode,eamodio/vscode,microsoft/vscode,eamodio/vscode,eamodio/vscode,Microsoft/vscode,Microsoft/vscode,microsoft/vscode,eamodio/vscode,microsoft/vscode,microsoft/vscode,Microsoft/vscode,microsoft/vscode,Microsoft/vscode,microsoft/vscode,eamodio/vscode,eamodio/vscode,microsoft/vscode,microsoft/vscode,Microsoft/vscode,Microsoft/vscode,eamodio/vscode,Microsoft/vscode,microsoft/vscode,eamodio/vscode,Microsoft/vscode,microsoft/vscode,eamodio/vscode