Revise FORM-as-function interface in erc-button-alist

Revise FORM-as-function interface in erc-button-alist

* lisp/erc/erc-button.el (erc-button-alist): Remove redundant "<URL:
foo>" entry, which adds nothing beyond highlighting the surrounding
bookends at the expense of doubling up on face properties for no
reason.  Revise the FORM-as-function interface by removing the dynamic
binding of face options and treating all implementers as replacements
for `erc-button-add-button'.
(erc-button--maybe-warn-arbitrary-sexp): Make more robust by having it
handle all accepted FORM types other than booleans.
(erc-button-add-buttons-1): Rework to only check FORM field once.
(erc-button--substitute-command-keys-in-region,
erc-button--display-error-with-buttons): Rename former as latter and
change signature to conform to new `erc-button-add-buttons' interface.
(erc-button--display-error-notice-with-keys): Call renamed helper.
* test/lisp/erc/erc-button-tests.el (erc-button-alist--url,
erc-button-tests--form, erc-button-tests--some-var,
erc-button-tests--erc-button-alist--function-as-form,
erc-button-alist--function-as-form,
erc-button-tests--erc-button-alist--nil-form,
erc-button-alist---nil-form): Add tests and helpers.  (Bug#60933)