Use diffhtml to update rendered html

[CaptainCodeman]

Uses diffhtml to patch the rendered HTML to prevent img, video, and iframe content being re-loaded on every edit.

Closes #162

[BearToCode]

Hi! I like this idea, however I've tested it and it breaks the component plugin (as well as any plugin using the carta-render callback), as using a separate callback to edit the final HTML interferes with diffhtml.

Maybe there's another approach for just "recycling" the HTML for images and videos, and not everything else?

[CaptainCodeman]

I haven't used either of those - TBH I thought that was the spot where the renderedHtml was written to the DOM, but I guess not?

[BearToCode]

It is, however those plugins work by manipulating the generated HTML (i.e. the one Renderer.svelte sees) every time it renders. Doing so breaks diffhtml, which would otherwise work.

I'm not really sure how this could be avoided. Maybe there's a way to sort of cache videos and images, to prevent the flickering visible in #162 ?