feat: Adds negative z-index rule to stylelint

[orangevolon]

Description

Following up the AWSUI-53531 issue, this rule is added to remind the devs to avoid negative z-index values.

Why not to use an existing stylelint rule?

1. The existing stylelint rule is stylelint-z-index-value-constraint which despite having min and max values, doesn't support negative z-index values.
2. Custom error message to explain "why" this value should not be used.
3. Expanding it in the future to cover other z-index recommendations.

Related links, issue #, if available: AWSUI-55594

How has this been tested?

1. Unit tests.
2. Verified the build artifacts with the components repo.

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.

Testing

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

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[codecov]

Codecov Report

Attention: Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.44%. Comparing base (9d4dc30) to head (0a3f65d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/stylelint/index.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   92.43%   93.44%   +1.01%     
==========================================
  Files           4        5       +1     
  Lines         185      229      +44     
  Branches       40       47       +7     
==========================================
+ Hits          171      214      +43     
- Misses         13       14       +1     
  Partials        1        1              

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