feat: --dirs option.

[knightedcodemonkey]

• Adds --dirs option.
• Updates dependency babel-dual-package.
• Updates README.

BREAKING CHANGE:

• Renames the dual build dir for CJS-first projects from mjs to esm. Update your project's exports before publishing after building with duel.

[codecov[bot]]

Codecov Report

Merging #9 (3f75f54) into main (9bff3f7) will increase coverage by 0.24%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   96.47%   96.72%   +0.24%     
==========================================
  Files           3        3              
  Lines         284      305      +21     
==========================================
+ Hits          274      295      +21     
  Misses         10       10              

Files Changed	Coverage Δ
src/duel.js	93.15% <100.00%> (+0.72%)	:arrow_up:
src/init.js	100.00% <100.00%> (ø)

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