Improve the Svelte web app

[dabreegster]

• [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