Allow ERC's module toggles access to the prefix arg

Allow ERC's module toggles access to the prefix arg

* lisp/erc/erc-common.el (erc--module-toggle-prefix-arg): Add internal
variable for preserving the `arg' passed to a module's minor-mode
toggle, which was previously discarded.  Doing this lets modules that
are more interactive in nature overload their mode toggles with
alternate behaviors.
(define-erc-module): Bind `erc--module-toggle-prefix-arg' to the `arg'
parameter, which is normally defined inside a `define-minor-mode' body
form.
* test/lisp/erc/erc-tests.el (define-erc-module--global,
define-erc-module--local): Expect activation body to be wrapped by a
let form binding `erc--module-toggle-prefix-arg'.  (Bug#63595)