cask/audit: detect tag from URL

[SeekingMeaning]

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The current method of using cask.version as a tag is not good enough as many repositories add a prefix such as v to their tags.

This PR adds two new methods to SharedAudits: github_tag_from_url and gitlab_tag_from_url, which can be used by brew cask audit to detect a version tag from GitHub/GitLab URLs.

Admittedly, GitLab uploads don't seem to include the tag in their URLs, but this is not a major problem as only two casks are downloaded from GitLab. (See https://github.com/Homebrew/homebrew-cask/search?q=%22gitlab.com%22) Perhaps as a workaround we could hardcode the tags for these two casks?

Related: Homebrew/homebrew-cask/pull/88936

[reitermarkus]

Thanks, @SeekingMeaning!