Add ESM support

[Lodin]

Resolves #93

[Lodin]

@calebdwilliams, I wonder what would be better: to use .mjs in module field or to remove module field completely in favor of exports field? What do you think?

[codecov-commenter]

Codecov Report

Merging #104 (f4208ad) into main (e6c5177) will not change coverage.
The diff coverage is n/a.

❗ Current head f4208ad differs from pull request most recent head b4b294e. Consider uploading reports for the commit b4b294e to get more accurate results

@@           Coverage Diff           @@
##             main     #104   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files           5        5           
  Lines         189      189           
  Branches       32       32           
=======================================
  Hits          174      174           
  Misses          4        4           
  Partials       11       11           

Flag	Coverage Δ
unittests	92.06% <ø> (ø)

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[calebdwilliams]

@Lodin in my experience exports is spotty until everything adopts TypeScript@4.5. I’d definitely mark that as a to-do but maybe not right away unless it’s absolutely needed.

[calebdwilliams]

Also @Lodin let me know when this is final and I’ll try it out locally.