Add the loading content attribute and IDL for <map>

[Malvoz]

UCR link: Maps4HTML/HTML-Map-Element-UseCases-Requirements#232

Lazy-loading is a widely used technique to improve performance and help users conserve bandwidth. The HTML spec defines the loading attribute for images and iframes (I'm aware of plans to support video as well).

I believe it'd make the most sense to have <map loading="lazy"> defer all fetches from <layer src>, but probably also other inline MapML elements that fetch resources.

This feature is not essential to getting maps into browsers (and is probably a bit more complex implementation than that of <img loading="lazy"> or <iframe loading="lazy">). For that reason, perhaps it's best to defer this feature until the spec has reached an appropriate Maturity Level on the W3C Recommendation Track.