Use ES Modules

[adamwolf]

Desiderata

• Answer Adam's question: Can we get this as an ESM module?

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Cognata

• https://github.com/beeminder/beeminder/issues/3888
• https://github.com/beeminder/beeminder/issues/3303
• Can't run gulp #291
• https://hacks.mozilla.org/2018/03/es-modules-a-cartoon-deep-dive/

Verbata: infrastructure, javascript modules, gulp, compiling, ES modules, ESM, CommonJS vs ESM, CJS vs ESM,