Allow set --query to check for pathvarness (#8494)

Currently,

    set -q --unpath PATH

simply ignores the "--unpath" bit (and same for "--path").

This changes it, so just like exportedness you can check pathness.

CHANGELOG.rst

@@ -75,6 +75,7 @@ Scripting improvements
 - ``math`` now correctly prints negative values and values larger than ``2**31`` when in hex or octal bases (:issue:`8417`).
 - ``dirs`` always produces an exit status of 0, instead of sometimes returning 1 (:issue:`8211`).
 - ``cd ""`` no longer crashes fish (:issue:`8147`).
+- ``set --query`` can now query whether a variable is a pathvar via ``--path`` or ``--unpath`` (:issue:`8494`).
 
 Interactive improvements
 ------------------------