This PR adds the `TldrawImage` component that displays a tldraw snapshot
as an SVG image.

## Why
We've seen requests for this kind of thing from users. eg: GitBook, and
on discord:
<img width="710" alt="image"
src="https://github.com/tldraw/tldraw/assets/15892272/3d3a3e9d-66b9-42e7-81de-a70aa7165bdc">
The component provides a way to do that.
This PR also untangles various bits of editor state from image
exporting, which makes it easier for library users to export images more
agnostically. (ie: they can now export any shapes on any page in any
theme. previously, they had to change the user's state to do that).
## What else
- This PR also adds an **Image snapshot** example to demonstrate the new
component.
- We now pass an `isDarkMode` property to the `toSvg` method (inside the
`ctx` argument). This means that `toSvg` doesn't have to rely on editor
state anymore. I updated all our `toSvg` methods to use it.
- See code comments for more info.
## Any issues?
When you toggle to editing mode in the new example, text measurements
are initially wrong (until you edit the size of a text shape). Click on
the text shape to see how its indicator is wrong. Not sure why this is,
or if it's even related. Does it ring a bell with anyone? If not, I'll
take a closer look. (fixed, see comments --steve)
## Future work
Now that we've untangled image exporting from editor state, we could
expose some more helpful helpers for making this easier.
Fixes tld-2122
### Change Type
- [x] `minor` — New feature
[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version
### Test Plan
1. Open the **Image snapshot** example.
2. Try editing the image, saving the image, and making sure the image
updates.
- [ ] Unit Tests
- [ ] End to end tests
### Release Notes
- Dev: Added the `TldrawImage` component.
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Co-authored-by: Steve Ruiz <steveruizok@gmail.com>