Improve the Svelte web app

[dabreegster]

• 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
• Changing the test / imported area should discard all lane edits
• The download button shouldn't just name things osm.xml
• 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