feat: correct module systems by file extension.

[knightedcodemonkey]

• Ensures the correct module system by file extension for each build, despite the difficulties in trying to do this with tsc.
• Allows a peerDependency on typescript 4.

[codecov]

Codecov Report

Merging #5 (6f91a89) into main (bfd59a9) will increase coverage by 0.67%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   96.00%   96.67%   +0.67%     
==========================================
  Files           3        3              
  Lines         250      301      +51     
==========================================
+ Hits          240      291      +51     
  Misses         10       10              

Files Changed	Coverage Δ
src/duel.js	93.90% <100.00%> (+2.75%)	⬆️

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