Allow default values in 'map-let' and the pcase 'map' form

Allow default values in 'map-let' and the pcase 'map' form

* lisp/emacs-lisp/map.el (map-let, map)
(map--make-pcase-bindings): Add a third argument for specifying a
default value, like in 'map-elt'. (Bug#49407)

* lisp/emacs-lisp/map.el (map--make-pcase-bindings): Clarify that keys
that aren't found aren't ignored, they actually get the value
nil (unless the new default value is given).  The overall pattern can
still fail to match if the sub-pattern for the unfound key doesn't
match nil.

* test/lisp/emacs-lisp/map-tests.el (test-map-let-default)
(test-map-plist-pcase-default, test-map-pcase-matches): Add tests,
including for the above item.