Create Classy library dependency tests that can be used for all themes, and verify by providing an Umami-specific classy/dropbutton

Problem/Motivation

As stated in #3050389: Remove dependency to Classy from core themes, Classy will be moved to contrib before Drupal 9 and we have to remove dependencies on Classy from all core themes.

Part of this process includes creating theme-specific copies of all Classy libraries to core themes. This ensures Classy can be removed without impacting those themes.

Because this is the first of several issues for copying a Classy library to Umami, there are no tests for this yet.

Also because this is the first of several similar issues, this will also be used to collect feedback about the process and naming conventions, with the goal of making the additional issues as easy as possible to patch and review.

Proposed resolution

• Make an Umami-specific copy of classy/dropbutton This includes:
  • The library should be called classy.dropbutton, using the classy. prefix to make it clear that it is copied from Classy, not custom to Umami
  • Umami should extend core/drupal.dropbuttonso the end result behaves identically to Classy's extending of the library.
  • The CSS file will be located in css/classy/components. As a rule, copied assets from Classy should go in a classy subdirectory of that asset type, but otherwise maintain the directory structure from classy (such as going in a components or theme directory)
• Add a test class that confirms the copy is successful, and build it so it is easily capable of testing other libraries when they are copied to a theme that previously depended on the Classy library.
• Manually test in Umami by comparing elements that use the dropbutton css before-patch and after-patch, and provide screenshots.

Remaining tasks

Currently all the steps above.

User interface changes

no

API changes

no

Data model changes

no

Release notes snippet

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