feat: support *_instance_of in RSpec/Rails/MinitestAssertions

[G-Rath]

Related to rubocop/rubocop-rspec_rails#7

I've not included this in the cops documentation as I figure ultimately documenting every single form of matcher translation could end up being pretty big and this is a straightforward one - happy to add it though if folks disagree/don't mind the length.

I've also refactored the assertion classes so they extend from a basic parent that handles the generation of the actual matchers - I've kept this as a separate commit for easier review and in case folks want that landed in a different PR (or not at all).

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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.

[G-Rath]

I went with this form since it's the grammatically correct version and the "original" (be_instance_of is an alias)

[pirj]

Nice refactoring and addition! Thank you

[ydah]

Thank you!