Encoding about as slow as `JSON.stringify`

Consider this a companion for #114 to also challenge this claim from the README (and the similar claim from the pbf README):

> Very fast encoding and decoding — even faster than native JSON parse/stringify.

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Namely, this is our use-case: https://github.com/maplibre/maplibre-gl-js/issues/106#issuecomment-802834969

In Chrome, for me, `JSON.stringify` and `geobuf.encode`, seem to take about ~200ms each.
I did not try other browsers yet.

*(Unfortunately I can't share my actual GeoJSON, but if we decide to move forward with trying to make geobuf work for us, then I'll likely construct a similar test case; our GeoJSON is described in the linked issue though)*

Start by constructing a representative GeoJSON case from the use case described in the linked MapLibre issue, then benchmark geobuf.encode against JSON.stringify in Chrome and other browsers. Done means the comparison is reproducible and shows whether the README performance claim should be revised or the encoding implementation investigated.

Written by the indexing model from the issue text.