♻️ [Amp story shopping] Attachment as element child

[processprocess]

Refactor to use an element child instead of class inheritance.
This reduces bundle size by removing the import of AmpStoryPageAttachent as well as potential technical debt from a third level of inheritance.
(draggable-drawer > page-attachment > shopping-attachment).

[amp-owners-bot]

Hey @gmajoulet! These files were changed:

extensions/amp-story-shopping/0.1/amp-story-shopping-attachment.css
extensions/amp-story-shopping/0.1/amp-story-shopping-attachment.js
extensions/amp-story-shopping/0.1/test/test-amp-story-shopping-attachment.js
extensions/amp-story/1.0/amp-story-draggable-drawer.js

Hey @newmuis! These files were changed:

extensions/amp-story/1.0/amp-story-draggable-drawer.js

[coreymasanto]

Out of curiosity, how does this refactor reduce the bundle size?

Also, is there a technical concern about using inheritance here? This PR takes the existing structure of
BaseElement > draggable-drawer > page-attachment > shopping-attachment
and updates it to
BaseElement > draggable-drawer > page-attachment
BaseElement > shopping-attachment.
The updated structure seems like it may be more difficult to reason about during implementation

[processprocess]

It reduces the bundle size by removing the import of AmpStoryPageAttachent.

It can be implemented either way but the idea behind this approach is to favor composition over inheritance.
The only function we need to override is open. Other than that we will be switching out content within the attachment and editing the header.