JS decoder supporting on-demand geometry decoding

[springmeyer]

Currently in the JS decoder all geometries are fully materialized in memory for all features when a layer is decoded.

But we can on-demand/lazy parse geometries for radically more efficient decoding performance. This will result in less allocation overhead in Maplibre to have geometries requested on-demand.

Currently the JS benchmarks are written to compare on-demand geometry decoding of MVTs with https://github.com/mapbox/vector-tile-js against non-deferred, fully materialized geometry decoding of MLT. So the benchmarks are not really 🍏 to 🍏 until this feature is landed.

See also more context at #197