Option to toggle canvas rendering in entire Plot

With all marks supporting canvas rendering, it would be useful to be able to activate the canvas rendering for all marks in a plot, e.g. via

```svelte
<Plot canvas>
   <AreaY ... />
   <Line ... />
   <Rule ... />
</Plot>
```

For scenarios where multiple Plot instances are being used we could also add a `canvas` setting to the `svelteplot/defaults` context and respect it.