Update parser to treat file extensions as case-insensitive

[brandon93s]

This PR makes registration and resolution of parsers by file extension case-insensitive. Currently, to handle both .md and .MD or .jpg and .JPG you'd need to register both.

[codecov-io]

Codecov Report

Merging #743 into master will decrease coverage by 0.6%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #743      +/-   ##
==========================================
- Coverage   89.55%   88.95%   -0.61%     
==========================================
  Files          62       62              
  Lines        1992     1992              
==========================================
- Hits         1784     1772      -12     
- Misses        208      220      +12

Impacted Files	Coverage Δ
src/Parser.js	100% <100%> (ø)	⬆️
src/worker.js	90.9% <0%> (-9.1%)	⬇️
src/assets/CSSAsset.js	82.35% <0%> (-7.36%)	⬇️
src/Logger.js	43.54% <0%> (-6.46%)	⬇️
src/WorkerFarm.js	91.22% <0%> (-1.76%)	⬇️

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