Let format_offense add variable-width whitespace

[eugeneius]

Followup to #8069.

This allows expect_offense to be used when the dynamically generated part of the source code is not highlighted by the offense.

I couldn't find a spec in this repo that could be rewritten to use this option, but I have an open PR on RuboCop Rails that would benefit from it: https://github.com/rubocop-hq/rubocop-rails/pull/257/files#diff-d9cf3a5c58c7f3acbad8f9812ca8006bR70

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

I like the proposed change.

[koic]

This looks good to me. Can you rebase with the latest mater branch?

[eugeneius]

@koic done! ✅

[marcandre]

Thanks for completing what I should have done; I thought about it only after merging and didn't get back to it 👍

[koic]

Thanks for adding this feature. Can you apply it to https://github.com/rubocop-hq/rubocop-rails/pull/257/files#r436398573 ?