Improve the Svelte web app

- [x] Strongly consider porting to https://github.com/dimfeld/svelte-maplibre/ and contributing any necessary fixes there
- [ ] Change the CSS styling of the app to match the old version. This should hopefully be easy; each Svelte component gets its own style section that's scoped just there
- [ ] Clean up the old `debugClockwiseOrderingGeojson` call in favor of the per-intersection one
- [x] Changing the test / imported area should discard all lane edits
- [ ] The download button shouldn't just name things `osm.xml`
- [x] Use `index.js` or similar in `osm2streets-svelte`, so we can import many components from "one" place
- [ ] Fix the `popstate` hack. URL handling logic is currently split between 2 components.
- [ ] After recalculating in the lane editor, keep the focus on the same way
- [ ] Add back the debug layers feature