fix: move away from mjs extension to work in more environments

[AhmedAymanM]

Summary

moved away from using .mjs files extension for es-modules as it seems that there are some compatibility issues with It

Test plan

This code doesn't change any functionality, I ran the tests, and they all pass.

[netlify]

👷 Deploy request for xstyled pending review.
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🔨 Explore the source changes: 613e80a

[snaer-yolabs]

Nice work man! Hoping this goes out soon

[snaer-yolabs]

This should fix this issue
#264

[AhmedAymanM]

did you test it locally? @snaer-yolabs
I tested it with storybook and it fixed the issue for me.
I didn't try it with next.js or other tools though but I believe it should work nonetheless.

[snaer-yolabs]

No I did not test it locally @AhmedAymanM

[codecov]

Codecov Report

Merging #275 (613e80a) into main (4865316) will not change coverage.
The diff coverage is n/a.

❗ Current head 613e80a differs from pull request most recent head add2b2a. Consider uploading reports for the commit add2b2a to get more accurate results

@@           Coverage Diff           @@
##             main     #275   +/-   ##
=======================================
  Coverage   92.73%   92.73%           
=======================================
  Files          60       60           
  Lines        1252     1252           
  Branches      244      244           
=======================================
  Hits         1161     1161           
  Misses         91       91           

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[electroheadfx]

I have the same error under ViteJS/React install : http://host.electroheadfx.fr/efxbox/laurent/Capturedecran2021-06-19a18.34.32.png I have not any issue with styled-components, chakra-ui (so styled-system) packages with VITE