rg-isearch-menu should be autoloaded

[hmelman]

rg-isearch recommends configuring it via:

(define-key isearch-mode-map "\M-sr" 'rg-isearch-menu)

But this doesn't work alone, it fails on first use since rg-isearch isn't loaded. Putting an ;;;###autoload token before the transient-define-prefix call I think will fix it, and not make the user do a (require 'rg-isearch) or their own autoload call.

[dajva]

Fixed. Thanks for the report.