Fix: (level) Check number argument for preamble

This allows a query like this:

  (level <= (string-to-number (property PROPERTY)))

to proceed without signaling an error from ORG-QL--QUERY-PREAMBLE.

See #460.

Reported-by: Stewmath <https://github.com/Stewmath>

org-ql.info

@@ -1089,6 +1089,10 @@ File: README.info,  Node: 089-pre,  Next: 088,  Up: Changelog
      "PROPERTY-NAME" :inherit t)’.  (#460
      (https://github.com/alphapapa/org-ql/issues/460).  Thanks to
      Stewmath (https://github.com/Stewmath) for reporting.)
+   • Predicate ‘level’’s preamble optimizer allows expressions in place
+     of the numeric argument.  (See #460
+     (https://github.com/alphapapa/org-ql/issues/460).  Thanks to
+     Stewmath (https://github.com/Stewmath) for reporting.)
    • Reading of view settings from Org links in upcoming Emacs version.
      (#461 (https://github.com/alphapapa/org-ql/issues/461).  Thanks to
      Ola Nilsson (https://github.com/snogge) for help debugging, and for
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