Support a custom vacuum or optimize command on vec0 tables

[asg017]

If you DELETE a lot of vectors in a vec0 index, the vector chunks will contain a lot unused space. Sometimes it can be re-claimed when INSERT'ing new vectors (see #184), but there's no way to re-claim the disk space entirely.

So we should add a new "insert command" like FTS5 that allows you do manually fixup chunks and reclaim space:

INSERT INTO vec_items(vec_items) VALUES ('optimize');

This is easier said than done. This requires:

• Finding unused entries in older chunks
• Moving newer chunks into available old positions
• Deleting "empty" older chunks
• Making sure parition keys/metadata values stay the same

oh boy