Fix issue with select distinct

[davidmezzetti]

select distinct queries aren't being interpreted correctly. For example:

embeddings.search("select distinct id, text from txtai where similar('test')")

doesn't return the distinct id/text rows. There is most likely an issue with the query preprocessor and the distinct keyword.

In the meantime, the following workaround can be used:

embeddings.search("select distinct(id || text), id, text from txtai where similar('test')")