feat: support some "predicate" assertions in RSpec/Rails/MinitestAssertions

[G-Rath]

This implements some support for assert_predicate and co, focused on transforming usage with predicate methods (e.g. valid?) into their be_ equivalents.

Predicates that are not symbols or that don't end with ? are ignored - a follow up PR can expand support for other uses of assert_predicate.

I think technically this makes the autofix unsafe because assert_predicate uses send whereas RSpec uses send_public so you could assert a private method which would break when fixed, but I feel like that would generally be considered bad practice anyway and an easy mistake so arguably this could be useful?

Relates to rubocop/rubocop-rspec_rails#7

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Before submitting the PR make sure the following are checked:

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

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.

[pirj]

Thank you!
I agree that even though we deliberately suggest a potentially unsafe correction, it's for the greater good.

You're on fire with those assertion PRs 🔥