Add slot methods to a module instead of the component class itself

[ozydingo]

What are you trying to accomplish?

Addresses #2027:

• Define slot methods in a dynamic module so that slot name methods can be overridden with ability to call super

What approach did you choose and why?

• Inspired by ActiveRecord's naming of GeneratedAttributeMethods, I am choosing the name GeneratedSlotMethods
• Define a new module and set it to the const self::GeneratedSlotMethods in the inherited hook defined by ViewComponent::Slotable, similar to how ActiveRecord creates <model>::GeneratedAttributeMethods.
• include self::GeneratedSlotMethods in the same inherited hook
• define the <slot> and <slot>? methods on the module

TODO

• FIX: Use the inherited hook and not the included hook; ViewComponent::Slotable is included only once by ViewComponent::Base
• Add test to make sure independent components do not share methods
• Ensure component subclasses that define their own slots also inherit superclass slot methods
• Investigate other slottable dynamic methods (currently commented out)

Anything you want to highlight for special attention from reviewers?

• Added a step in CONTRIBUTING to bundle install, with the goal of installing appraisal (otherwise bundle exec appraisal install would fail)
• Running bundle exec appraisal install modified the gemfiles and I'm not sure why. I have no experience with appraisal. I imagine I will need to revert these changes, but I'm keeping them on the branch for now since if I revert I can't run tests, and also to ask about it.

[ozydingo]

@joelhawksley after a "brief" hiatus I cam back to this and got it in working order. Unfortunately the "Test compatibility with Primer ViewComponents" is failing, and if anyone had some bandwidth to help shine some light into that failure that would be hugely appreciated. I'll try to dig in when I get another chance but I'll be wandering in a lot of unfamiliar territory as I do! It's also pretty unclear to me, at a high level, why this change would specifically break the test_single_select_item_checked_via_keyboard_space test so I suspect the resolution will involve a lot of digging into those component specifics.

[joelhawksley]

Looks good to me! I'm going to defer to @BlakeWilliams for the review here ❤️

[BlakeWilliams]

LGTM! Thanks for this improvement!