This patch adds some libSQL vector extension functions such as
`vector()` and `vector_distance_cos()`, which can be used for exact
nearest neighbor search as follows:
```
limbo> SELECT embedding, vector_distance_cos(embedding, '[9, 9, 9]')
...> FROM movies ORDER BY vector_distance_cos(embedding, '[9, 9, 9]');
[4, 5, 6]|0.013072490692138672
[1, 2, 3]|0.07417994737625122
```
