Inject datetime values as <time> element

[nichtich]

<wd-entity id="Q42" property="P569" />

should result in

<time datetime="1952-03-11">1952-03-11</time>

so it can better be reused for instance with https://timeago.yarp.com/ to customize how date values are rendered. The precision of a datetime value also needs to be taken into account.