fix: support ESM version of postcss.config.js

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

Allow to load ESM configuration, tests are impossible due bugs and unstable ESM working in jest, so tested manually

Breaking Changes

No

Additional Info

Fixes #613

[codecov]

Codecov Report

Base: 93.14% // Head: 89.33% // Decreases project coverage by -3.80% ⚠️

Coverage data is based on head (c95cd70) compared to base (f1729f8).
Patch coverage: 44.44% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #614      +/-   ##
==========================================
- Coverage   93.14%   89.33%   -3.81%     
==========================================
  Files           5        5              
  Lines         321      347      +26     
  Branches      106      113       +7     
==========================================
+ Hits          299      310      +11     
- Misses         20       34      +14     
- Partials        2        3       +1     

Impacted Files	Coverage Δ
src/utils.js	85.10% <44.44%> (-5.33%)	⬇️

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[its-me-VOID]

postcss.config.mjs silently fail, as if the config wasn't specified.
I think this PR didn't bring support of esm module.

[its-me-VOID]

@alexander-akait cosmiconfig from version 8.2.0 support ESM config without patch, with the Async APi
https://github.com/cosmiconfig/cosmiconfig#loading-js-modules

[alexander-akait]

@its-me-VOID

postcss.config.mjs silently fail, as if the config wasn't specified.
I think this PR didn't bring support of esm module.

Can you provide reproducible test repo?