fix: properly support loader option

[tivac]

Description

@modular-css/rollup-rewriter advertises support for a loader option, but it wasn't actually pumped through to the different format rewriters.

Motivation and Context

I want to use a custom loader, but couldn't.

How Has This Been Tested?

Tests already existed, but just tested for the defaults. Now at least they're fully-testing that codepath.

Types of changes

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

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #549 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #549   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files          46       46           
  Lines        1026     1026           
  Branches      157      157           
=======================================
  Hits         1015     1015           
  Misses         11       11

Impacted Files	Coverage Δ
packages/rollup-rewriter/formats/system.js	88.88% <ø> (ø)	⬆️
packages/rollup-rewriter/formats/amd.js	88.88% <ø> (ø)	⬆️
packages/rollup-rewriter/formats/es.js	100% <100%> (ø)	⬆️

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