feat: add create_index to the async python API (#1052)

This also refactors the rust lancedb index builder API (and,
correspondingly, the nodejs API)

nodejs/src/lib.rs

@@ -16,6 +16,7 @@ use connection::Connection;
 use napi_derive::*;
 
 mod connection;
+mod error;
 mod index;
 mod iterator;
 mod query;