fix: use module field as esm hint for bundlers

[fallenoak]

This should ensure webpack and other bundlers prefer the esm version of the code.

Entrypoints by environment:

• Node.js in cjs mode: ambiguous main dist/index resolves to dist/index.js
• Node.js in legacy esm mode: ambiguous main dist/index resolves to dist/index.mjs
• Node.js in modern esm mode: exports.import resolves to dist/index.mjs
• webpack: module resolves to dist/index.mjs

[codecov]

Codecov Report

Merging #51 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #51   +/-   ##
=======================================
  Coverage   97.79%   97.79%           
=======================================
  Files           6        6           
  Lines         634      634           
  Branches       79       79           
=======================================
  Hits          620      620           
  Misses         14       14           

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