Support the `IS` and `ISNOT` operator on vec0 metadata filterins

[asg017]

Came up in Discord, some people might use the IS operator in certain metadata filters, like:

select *
from vec_items
where embedding match ?
  and k = 20
  and is_hidden is false -- not supported yet!

We only override the = operator, not IS.

Need to read up on the IS operator — is it always the same as the EQ operation?

// https://www.sqlite.org/vtab.html#the_xbestindex_method
#define SQLITE_INDEX_CONSTRAINT_ISNOT     69  /* 3.21.0 and later */
#define SQLITE_INDEX_CONSTRAINT_ISNOTNULL 70  /* 3.21.0 and later */
#define SQLITE_INDEX_CONSTRAINT_ISNULL    71  /* 3.21.0 and later */
#define SQLITE_INDEX_CONSTRAINT_IS        72  /* 3.21.0 and later */