Add file extensions for C++20 module interface units (microsoft#214800)

Add file extensions: C++20 module interface units

Author: powersagitar

extensions/cpp/package.json

@@ -30,9 +30,13 @@
         "id": "cpp",
         "extensions": [
           ".cpp",
+          ".cppm",
           ".cc",
+          ".ccm",
           ".cxx",
+          ".cxxm",
           ".c++",
+          ".c++m",
           ".hpp",
           ".hh",
           ".hxx",