Specify <mapcaption>, using <figcaption> as a model

[prushforth]

We think that using <figcaption> directly is not appropriate, since maps don't necessarily share semantics with figures. So, <mapcaption> may be an appropriate new element name. The relation to the parent map viewer element should be quite similar to that between <figcaption> and the parent <figure>, we think.

See :
Maps4HTML/HTML-Map-Element-UseCases-Requirements#260
and
https://maps4html.org/web-map-doc/docs/elements/map-caption/

Note that <figcaption> implements the HTMLElement WebIDL interface.

[prushforth]

Done