add svelte files

src/source.cpp

@@ -71,6 +71,8 @@ Glib::RefPtr<Gsv::Language> Source::guess_language(const boost::filesystem::path
       language = language_manager->get_language("cpphdr");
     else if(extension == ".ts" || extension == ".tsx" || extension == ".jsx" || extension == ".flow")
       language = language_manager->get_language("js");
+    else if(extension == ".svelte")
+      language = language_manager->get_language("html");
     else if(extension == ".vert" || // listed on https://github.com/KhronosGroup/glslang
             extension == ".frag" ||
             extension == ".tesc" ||