feat(rollup-rewriter): let loader option be a function

[tivac]

Description

Let the loader option be either a string or a function.

Motivation and Context

Allows for picking a file out of the bundle to be used, even if the output name is hashed by rollup.

How Has This Been Tested?

Added tests.

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 #667 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #667      +/-   ##
==========================================
+ Coverage   99.12%   99.12%   +<.01%     
==========================================
  Files          45       45              
  Lines        1138     1139       +1     
  Branches      173      174       +1     
==========================================
+ Hits         1128     1129       +1     
  Misses         10       10

Impacted Files	Coverage Δ
packages/rollup-rewriter/rewriter.js	100% <100%> (ø)	⬆️
packages/rollup-rewriter/formats/es.js	100% <100%> (ø)	⬆️
packages/rollup-rewriter/formats/amd.js	92.85% <100%> (ø)	⬆️
packages/rollup-rewriter/formats/system.js	91.66% <100%> (ø)	⬆️

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