Add new Rails/AssertNot cop

[composerinteralia]

Following the pattern of #5801, this cop imports a custom cop
from rails/rails#32605.

This cop checks for the use of assert !:

# bad
assert !foo

# good
assert_not foo

As in #5801, since assert_not is a method defined in ActiveSupport,
this cop belongs in the Rails namespace.

---

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 rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

👍