Feature/accordion slots

[bgilhome-now]

Issue:
A PR for the feature request at #1288

What I did

• Allowed passing accordion items' content & title via slots (default slot and named slot 'title' respectively)
• Allowed items prop to be undefined, auto-setting integer item IDs if slots are used instead

How to test

• See new 'Accordion using slots' story in the Storybook for the Accordion component

Checklist

For all PR's

• I've added relevant changes to the project Readme if needed.
• I've updated the documentation site as needed.
• I have added unit tests to cover my changes (if not applicable, please state why in a comment)

For new components only

• I have added a story covering all variants
• I have checked a11y tab in storybook passes
• Any events are emitted on the event bus

[dylankelly]

@bgilhome-now there are few implementation issues with this that we have raised a ticket internally to look at. We will look at this in the next release.

[bgilhome-now]

Thanks @dylankelly !

[waitingallday]

Released in #1327 using the child-component-wrapper pattern we've used in a few other places.

@bgilhome-now thanks for the PR 🙌 a sample of the required markup is included in the PR description above (two slots, one named).