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

[ozydingo]

What are you trying to accomplish?

Addressed #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.