feat(webpack): add mjs to babel-loader

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Close #6688

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #7178 into dev will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #7178   +/-   ##
=======================================
  Coverage   62.82%   62.82%           
=======================================
  Files          84       84           
  Lines        3400     3400           
  Branches      928      928           
=======================================
  Hits         2136     2136           
  Misses       1017     1017           
  Partials      247      247           

Flag	Coverage Δ
#unittests	62.82% <ø> (ø)

Impacted Files	Coverage Δ
packages/webpack/src/config/base.js	0.00% <ø> (ø)

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