feat: stepper: adds show active step index prop

[dkilgore-eightfold]

SUMMARY:

Ensures there is an option to show the numerical index for the active timeline step rather than an icon.

showActiveIndex.mp4

smallSize.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 Timeline stories behave as expected.

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@62f433f). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

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  Hits            ?    13877           
  Misses          ?     2415           
  Partials        ?       67           

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