Enable lazy loading for URLs on use href

[lfre]

https://svgwg.org/svg2-draft/struct.html#UseElement

The loading attribute enables lazy loading for images and iframes. Likewise, the <use> tag could use this functionality to enable deferring the loading of SVGs as a URL. E.g:

<svg>
	<use href="/path/to/my.svg" loading="lazy" />
</svg>

What solutions exist today?

Like polyfills for lazy-loading, an Intersection Observer mechanism can swap a data-href attribute to href when approaching in view.

[Malvoz]

Relatedly there's a request to support loading on <image> in #817.