conditionally use star-history dark theme (#3461)

This PR makes the "github star history" image on the readme use a dark
image if the user's preference is for dark mode.

### Change Type

<!-- ❗ Please select a 'Scope' label ❗️ -->

- [ ] `sdk` — Changes the tldraw SDK
- [ ] `dotcom` — Changes the tldraw.com web app
- [x] `docs` — Changes to the documentation, examples, or templates.
- [ ] `vs code` — Changes to the vscode plugin
- [ ] `internal` — Does not affect user-facing stuff

<!-- ❗ Please select a 'Type' label ❗️ -->

- [ ] `bugfix` — Bug fix
- [ ] `feature` — New feature
- [x] `improvement` — Improving existing features
- [ ] `chore` — Updating dependencies, other boring stuff
- [ ] `galaxy brain` — Architectural changes
- [ ] `tests` — Changes to any test code
- [ ] `tools` — Changes to infrastructure, CI, internal scripts,
debugging tools, etc.
- [ ] `dunno` — I don't know


### Test Plan

1. Add a step-by-step description of how to test your PR here.
2.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

updates the star-history image in the README to conditionally show a
dark theme image based on the user's `prefers-color-scheme`

README.md

@@ -77,7 +77,17 @@ Please see our [contributing guide](https://github.com/tldraw/tldraw/blob/main/C
 ## Star History
 
 <a href="https://star-history.com/#tldraw/tldraw">
-  <img src="https://api.star-history.com/svg?repos=tldraw/tldraw&type=Date" alt="Star History Chart" width="100%" />
+	<picture>
+	  <source
+	    media="(prefers-color-scheme: dark)"
+	    srcset="https://api.star-history.com/svg?repos=tldraw/tldraw&type=Date&theme=dark"
+	  />
+	  <source
+	    media="(prefers-color-scheme: light)"
+	    srcset="https://api.star-history.com/svg?repos=tldraw/tldraw&type=Date"
+	  />
+	  <img src="https://api.star-history.com/svg?repos=tldraw/tldraw&type=Date" alt="Star History Chart" width="100%" />
+	</picture>
 </a>
 
 ## Contact