Add typing for Cask#url and fix detected issues

[Bo98]

Most runtime TypeErrors indicate we have failed to type things correctly so that those issues can be detected statically (typically due to T.untyped).

This PR fixes this error:

Error: exception while auditing betterdisplay: Parameter 'url': Expected type String, got type Cask::URL with value #<Cask::URL::DSL:0x00000001...lay/", :user_agent=>:default}>
Caller: /opt/homebrew/Library/Homebrew/cask/audit.rb:768
Definition: /opt/homebrew/Library/Homebrew/utils/shared_audits.rb:187 (SharedAudits.github_tag_from_url)

And also fixes the underlying issue so that this issue can be detected in the future via brew typecheck

[MikeMcQuaid]

Thanks @Bo98!