[Fix #10204] Add new Lint/UnsafeGlobalVariables cop

[dmarcoux]

This cop looks for uses of unsafe global variables.

Since this is my first contribution, I've based this cop on Style/GlobalVars to follow coding conventions as best as possible. I'm unsure if it makes sense to introduce a new cop for this, since Style/GlobalVars is quite similar.

Fixes #10204

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[dmarcoux]

I don't really understand why ci/circleci: ruby-head-ascii_spec and ci/circleci: ruby-head-spec are failing. Could someone help me out with this?

[koic]

I don't really understand why ci/circleci: ruby-head-ascii_spec and ci/circleci: ruby-head-spec are failing. Could someone help me out with this?

These CI matrices have been resolved by #10213. Can you rebase with the latest master branch?

[bbatsov]

I'm unsure if it makes sense to introduce a new cop for this, since Style/GlobalVars is quite similar.

Yeah, that's a good point. Perhaps this can be a feature of the other cop controlled by a boolean flag. One more thing - we should clearly document what "unsafe" means exactly.

[dmarcoux]

Thank you @bbatsov for the feedback. I don't have time to work on this now and in the foreseeable future though. Anybody can pick this up where I left it.

[bbatsov]

No rush.