Add lesser-known ILPROJ extension to MSBUILD

Many ILPROJ files occur, for example, in https://github.com/dotnet/runtime. These are rather like CSPROJ files. For some further discussion of ILPROJ, see dotnet/sdk#16690.

languages.json

@@ -4544,6 +4544,7 @@
       "vcproj",
       "vcxproj",
       "vcxproj.filters",
+      "ilproj",
       "myapp",
       "props",
       "rdlc",