Support .ipp files

src/dds/source/file.cpp

         ".hpp",
         ".hxx",
         ".inl",
+        ".ipp",
     };
     assert(std::is_sorted(header_exts.begin(), header_exts.end()));
     static std::vector<std::string_view> source_exts = {
         ".cpp",
         ".cxx",
     };
+    assert(std::is_sorted(header_exts.begin(), header_exts.end()));
     assert(std::is_sorted(source_exts.begin(), source_exts.end()));
     auto leaf = p.filename();