Flatgeobuf extension based on `GeoJSONSource.svelte`

[spwoodcock]

• Flatgeobuf is a performant vector format, with HTTP RANGE support for bbox filtering, and decoding to GeoJSON.
• I thought it could be nice to provide an extension here that handles the fgb --> geojson decoding, taking the bbox (for viewport) into account, then simply passing through to GeoJSONSource.svelte.
• I did something similar for svelte-maplibre (I imagine it would be simpler in this lib without the workarounds for layer reconstruction).

Happy to contribute this if it's wanted 😄