fix: ensure sourceMappingUrl is correct

[adamstankiewicz]

Description

Ensures sourceMappingUrl in generated *.min.css is relative to the same directory such that it can be loaded when requested.

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[edx-semantic-release]

🎉 This PR is included in version 21.0.0-alpha.35 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[edx-semantic-release]

🎉 This PR is included in version 22.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[openedx-semantic-release-bot]

🎉 This PR is included in version 23.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[openedx-semantic-release-bot]

🎉 This PR is included in version 23.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.58%. Comparing base (a8069b0) to head (7830735).
Report is 880 commits behind head on alpha.

Additional details and impacted files

@@           Coverage Diff           @@
##            alpha    #2352   +/-   ##
=======================================
  Coverage   91.58%   91.58%           
=======================================
  Files         214      214           
  Lines        3519     3519           
  Branches      852      852           
=======================================
  Hits         3223     3223           
  Misses        288      288           
  Partials        8        8           

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