Specialize default-get for alist widgets (Bug#63290)

Specialize default-get for alist widgets (Bug#63290)

* lisp/wid-edit.el (widget-list-default-get)
(widget-alist-default-get): New functions.
(list, alist): Use it.
* test/lisp/cus-edit-tests.el (cus-edit-test-bug63290-option)
(cus-edit-test-bug63290-option-2): New test options.
(cus-edit-test-bug63290): New test.
* test/lisp/wid-edit-tests.el (widget-test-alist-default-value-1)
(widget-test-alist-default-value-2)
(widget-test-alist-default-value-3)
(widget-test-alist-default-value-4): New tests.