Announce zoom

[kevinkim31]

closes #664

• Update Web-Map-Custom-Element to audibly render zoom in output by default i.e. on by default
• Update MapML-Extension to reflect the fact that the option to audibly render zoom is on by default, provide option to turn it off. "Announce movement for screen readers" -> "Announce Zoom"
• Update Web-Map-Doc.
• Change the name of this experiment. We should add the pan/zoom control to that map, as well as a <map-caption>Paris, the City of Light</map-caption>

[prushforth]

@Malvoz no rush but if you could have a listen etc to this, please. I find that announce movement may not be too useful in its current form, but announce zoom might be a bit more useful as an update to that functionality. I had the notion that it would be cool to calculate an approximate scale ratio for the (centre of the) current map view and announce that, e.g. "One centimetre to 47 metres" etc. I guess we would have to add a visual scale bar control too if we did that, but the zoom levels themselves aren't too relatable to anyone just yet, similar to actual coordinates of any kind (that's computer language, not human language).

[prushforth]

See checklist

[prushforth]

Merging to keep the process going; feel free to comment when possible. Thanks!