livecheck: add RuboCops

[nandahkrishna]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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• Have you successfully run brew tests with your changes locally?

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This PR creates an audit and adds RuboCops for livecheck.

Audit

Check whether livecheck is able to find versions (doesn't error out) for formulae with livecheck blocks defined.

RuboCops

1. Ensure that skipped formulae do not have additional livecheck information such as url, regex, etc.
2. Ensure that a url is provided.
3. Ensure that a formula URL is referenced by a symbol (:head, :stable, :homepage).
4. Ensure that regexes contain \.t and not \.tgz or \.tar\.gz, etc.
5. Ensure that a regex is provided for strategy :page_match.
6. Ensure that regexes are case-insensitive (except formulae in allowlist).
7. Ensure that the regex is enclosed within parentheses.

I haven'[t included the cop for counting number of capturing groups – I wasn't able to write a regex that was able to do this, and I couldn't find a method/function to count capturing groups without matching.

The regex for the extension checking needs to be verified. Here's a more convoluted version I wrote that matches all possible tar extensions (I think): https://rubular.com/r/d5uJPJIONM63VE

It is possible that I've missed some important points in this PR description, but I will answer any questions as they arise. Looking forward to your reviews and suggestions everyone. Thanks!

CC @samford

[nandahkrishna]

Ah, I'd forgotten to mention: CI is 🔴 because of some recent updates to livecheck URLs in homebrew/core, which violate a cop this PR introduces. I'll create a PR to fix those issues.

[nandahkrishna]

With the latest commit, some changes have been made:

• livecheck_formula now accepts a hash containing the arguments (for the args parameter), and this is converted into an OpenStruct to be able to access the arguments via methods. As a result the LivecheckArgs class I'd earlier added is no longer required.

• Fixed spelling/comment error(s) and incorporated review suggestions.

[nandahkrishna]

The tests I added pass locally (brew tests --online --only dev-cmd/audit), but this failure seems to be related to the general issue of livecheck network tests not working on CI. If it's okay, I'll remove that particular test alone for now, and add it once we've arrived at a solution for the failures.

[MikeMcQuaid]

A few nits but otherwise looks great, nice work!

[nandahkrishna]

I've made all the changes @MikeMcQuaid, let me know if this is good to merge. I'll create a separate PR to address the livecheck puts suggestion and audit.