Accordion and tab-content missing expandedClass

Problem/Motivation

The accordion component does not open or close after theme generation.The accordion.scss uses .is-open to show the content panel, but accordion.js calls A11yAccordion without expandedClass, so the library defaults to toggling .is-expanded a class the CSS never responds to.

Steps to reproduce

1. Generate a theme from the Prototype Starterkit
2. Build assets (npm install && npm run build)
3. Add an accordion to a page and click a trigger content panel doesn't display

Proposed resolution

Pass expandedClass: 'is-open' to sync the JS with the SCSS

 new A11yAccordion(accordion, {
   triggerSelector: '.c-accordion__trigger[aria-controls]',
-  contentSelector: '.c-accordion__content',
+  contentSelector: '.c-accordion__content',
+  expandedClass: 'is-open',
 });