feat: accordion: deep accessibility updates

[dkilgore-eightfold]

SUMMARY:

• Removes multiple click target concern by moving overall clickable area inside the header, under the content via zIndex
• Updates focus-visible styles to work with the new layout
• Accounts for custom interactive content via selectors, such that it remains selectable or clickable
• Checks if custom content is a string, wraps it in a span so it remains selectable
• Removes a duplicate {...expandButtonProps} attribute
• Updates unit tests
• Updates stories

Lighthouse report:

Axe report:

JIRA TASK (Eightfold Employees Only):

ENG-87358

CHANGE TYPE:

• Bugfix Pull Request
• Feature Pull Request

TEST COVERAGE:

• Tests for this change already exist
• I have added unittests for this change

TEST PLAN:

Pull the PR branch and run yarn and yarn storybook. Verify the Accordion stories behave as expected.

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