Define missing groups for ERC modules

Define missing groups for ERC modules

* lisp/erc/erc-imenu.el (erc-imenu): Define group even though there
aren't any associated faces or user options because `imenu' is a
global module, and `define-erc-module' calls `define-minor-mode',
which needs a group for its mode variable.  This should have been
handled by bug#60954 but wasn't.
* lisp/erc/erc-sound.el (erc-sound): Move group definition above
module definition.
* lisp/erc/erc-spelling.el (erc-spelling): Define group.