feat: Steps connector lines display

[pan-kot]

Description

The PR introduces new API for Steps component:

interface StepsProps {
  // ...
  
  /**
   * Determines whether the connector lines between steps are displayed. The following variants are available:
   * * `default` - Shows connector lines between consecutive steps.
   * * `none` - Hides the connector lines between steps.
   */
  connectorLines?: "default" | "none";
}

<Steps 
  steps={[...]}
  connectorLines="none"
/>

This change is synonymous with the existing Tree view API that declares connectorLines: "vertical" | "none". While there are only two options for now, the union allows us to add more connector lines variants in the future.

Rel: AWSUI-62150

How has this been tested?

• New unit tests
• New permutations

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.59%. Comparing base (4d2e475) to head (7b1425d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4724      +/-   ##
==========================================
- Coverage   97.59%   97.59%   -0.01%     
==========================================
  Files         950      950              
  Lines       30664    30692      +28     
  Branches    11242    11257      +15     
==========================================
+ Hits        29928    29955      +27     
- Misses        689      690       +1     
  Partials       47       47              

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