fix: source map generation for cssnano

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #134

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #135 (c2d5321) into master (b7304ed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #135   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files           4        4           
  Lines         252      252           
  Branches       99       99           
=======================================
  Hits          242      242           
  Misses         10       10           

Impacted Files	Coverage Δ
src/utils.js	91.30% <ø> (ø)

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