org-ql does not support default Org priorities when sorting by priority

org-ql does not support default Org properties when querying by priority, which I wish it wasn't the case but is well documented and can be easily worked around combining different `priority` predicates (e.g., `(priority >= "B")` could be rewritten as `(not (priority "C"))`.

But it also doesn't support Org priorities when sorting by priority, and I think that's a bug, or at least it's not documented and I can't think of a reason for it to be that way, nor an easy workaround without using a custom sorting predicate. The fix is very simple and I've done it in my fork: https://github.com/igmacs/org-ql/commit/568e03b21e3532e61055cb8e37339ed514bdaf8b.