Expand `Capybara/NegationMatcher` to support `have_content`

[OskarsEzerins]

Added content to the list of matchers for Capybara/NegationMatcher. It allows said cop to check for not_to have_content, correct it to to have_no_content and vice versa.

have_content is not included in Capybara list of have_no_.. matchers but is rather aliased to have_no_text. (source). I'm not sure if thats perhaps a problem for this PR`s change. Maybe a different approach has to be taken due to above said.

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with main (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [ ] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[ydah]

@OskarsEzerins Could you rebase to the latest main branch?

[ydah]

@OskarsEzerins Thank you for doing a great work!