Re-claim deleted vector spaced in vec0 chunks more comprehensively

[asg017]

If you delete a row from a vec0 table, then the vector's space in the chunk will be "empty". If you insert a new row right after, sqlite-vec will only re-claim the space if the latest available chunk has an open source (ie if the deleted vector was in the last ~1024 inserted vectors).

We should make it so that when a chunk is full, we check previous chunks to see if there are empty slots, and insert there instead.

Doing this in a performant way will be difficult, it may make INSERTs on large tables less performant. Maybe a vec_popcount() SQL functions could make this easier? idk