Merge primer/rails_forms into primer/view_components - #1238

Purpose

Last quarter we made the conscious decision to home all the forms framework stuff in its own repo called https://github.com/primer/rails_forms with the acknowledgement that doing so enabled us to move faster in the short-term but that the code should ultimately live in primer/view_components. We also decided it made sense to rename primer/view_components to primer/rails, since the repo would no longer only contain components, but also forms-related code.

As you know, we decided to halt work on the forms framework this quarter, but would like to make good on our promise to unite the two repositories to enable other use-cases (eg. autocomplete). I've written up an issue delineating what I believe to be the concrete steps we need to take to achieve those goals: https://github.com/github/primer/issues/1143 (staff-only).

This PR is the result of tackling the first and second items on the list. We won't be attempting the other items on the list until later in the quarter.

Changes

In addition to copy/pasting all the files from PRF into PVC, making sure the tests run as expected, and exposing the documentation for TextField, I made several other facilitating changes:

1. Dropped support for Ruby 2.6 and Rails 5.2. It is fairly common practice in the Ruby world to support the last three minor versions, and the consensus on the team was that these older versions weren't important to keep around. We now support Ruby 2.7, 3.0, and 3.1, and Rails 6.1, 7.0, and main.
2. Added a number of tests missing from the original primer/rails_forms code (thanks for catching those, Simplecov).
3. Augmented the Lookbook's bin/dev script to accept a --debug option (ala dotcom) that starts all the non-web processes in the background and then runs the Rails server in the foreground. Doing so means debugging tools like binding.irb and binding.pry work as expected.
4. Upgraded form inputs to handle CSS classes with Primer::Classify the same way PVC components do. This means arguments like my: 4, etc can be passed to inputs.
5. Added documentation for the new TextField component, including examples.