fix: support ESM version of `postcss.config.js`

[alexander-akait]

This PR contains a:

• [x] 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 https://github.com/webpack-contrib/postcss-loader/issues/613

[codecov[bot]]

Codecov Report

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

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

```diff @@ 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](https://codecov.io/gh/webpack-contrib/postcss-loader/pull/614?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib) | Coverage Δ | | |---|---|---| | [src/utils.js](https://codecov.io/gh/webpack-contrib/postcss-loader/pull/614/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib#diff-c3JjL3V0aWxzLmpz) | `85.10% <44.44%> (-5.33%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib)

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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?