Use vtzero

Look again at using vtzero to encode and decode vector tiles instead of a tippecanoe-specific implementation

The issue names vtzero and tippecanoe's vector-tile encoding and decoding, but it does not identify files, tests, or an entry point. First locate the current tippecanoe-specific implementation and any encoding or decoding tests, then determine the scope of replacing it with vtzero and verify that existing vector-tile behavior remains covered.

Written by the indexing model from the issue text.