feature: svg block

[chrisparnin]

Give me an in-situ way to edit vector diagrams and you can have all my money.

https://twitter.com/gvwilson/status/1290617242678435840?s=20

Hmm, a simple step in this direction. Probably could just render a code block with svg code into a real svg.

```svg|{type:'svg'}
```

[chrisparnin]

As a hack, we only have require a simple svg block and render it in html.

```svg
...

Future version would probably upgrade this to a docable block, but would need to disable execution, but enable us to edit.