Bring back webm version of content/videos/overview-demo-setup-use-typ…

…ing-booster-emoji-picker.webm

It is used by typing-booster.appdata.xml which contains:

    <screenshot>
      <video container="mp4" codec="vp9">
        https://mike-fabian.github.io/ibus-typing-booster/videos/overview-demo-setup-use-typing-booster-emoji-picker.mp4
      </video>
      <caption>Typing booster demo video</caption>
    </screenshot>

And the documentation https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html says:

The video must be in a Matroska (.mkv) or WebM container and use either the VP9 or AV1 codec.