rubocops: disallow `libiconv` dependency in homebrew/core

`libiconv` is provided by macOS, so formulae in homebrew/core should
not depend on it directly. Add a `LibiconvCheck` cop to enforce this,
with a hardcoded exception for `neomutt`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>