[Fix #6630] Updated Style/CommentAnnotation to be able to handle multiword keyword phrases

[dvandersluis]

This change first refactors AnnotationComment to be an actual class modeling an annotation comment, and then updates it to be able to handle multiword phrases as annotation keywords.

With config:

Style/CommentAnnotation:
  Keywords:
    - TODO
    - TODO LATER

test.rb:1:1: C: [Correctable] Style/CommentAnnotation: Annotation keywords like TODO LATER should be all upper case, followed by a colon, and a space, then a note describing the problem.
# TODO LATER blah blah blah
  ^^^^^^^^^^^

Fixes #6630.

---

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.

[bbatsov]

Great work!