Allow formulae to suggest conflict workarounds

This came up in the discussion of #25382.

Sometimes, there are better alternatives to unlinking formulae for working around conflicts, like brewing conflicting formulae with certain options.  With this change, formulae can now suggest these alternatives with :suggests in their conflicts_with.

In the case of #25328, mutt.rb would be amended with:

```ruby
:suggest => 'brew signing-party with the --with-rename-pgpring option'
```

This is just a quick proof of concept.  Maybe :suggests should be an array of options:text tuples, allowing formulae to suggest multiple workarounds.