Add support for <em> and <code>

They were added in the spec in 2019

https://github.com/ximion/appstream/blob/a7e773cc9ce570dabe8d50f600b7b3dd4d55b98f/NEWS#L70

Some apps[ alredy use them](https://github.com/flathub/com.github.ztefn.haguichi/commit/d6ecad1b8af31633956b14bd5e94ed7852885338#r36562289)
