feat: stepper: adds timeline variant

[dkilgore-eightfold]

SUMMARY:

Adds Stepper timeline variant
Fixes 2 A11y bugs by placing aria-hidden="true" on non li elements in ul
Fixes minor typo in Stepper
Adds classNames from props to mergedClasses of Stepper
Updates Stepper snaps and UTs

stepper.mp4

JIRA TASK (Eightfold Employees Only):

ENG-39994

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 Stepper and Timeline stories behave as expected.

[codesandbox-ci]

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Latest deployment of this branch, based on commit 3e65469:

Sandbox	Source
React	Configuration
Vanilla	Configuration

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@65d51f0). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

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  Coverage        ?   84.79%           
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  Files           ?      784           
  Lines           ?    16361           
  Branches        ?     5672           
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  Hits            ?    13873           
  Misses          ?     2421           
  Partials        ?       67           

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[jhoward-eightfold]

do you need to export the component anywhere or is it already covered by wildcard export of Stepper?