feat: switch to ESM output

[wolfy1339]

BREAKING CHANGE: Output a single ESM bundle

[github-actions]

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[wolfy1339]

The types for the Fetch API don't allow headers to be passed as numbers, only as string. The docs also mention this https://developer.mozilla.org/en-US/docs/Web/API/Headers/Headers#init

I added a conversion since we accept numbers

[wolfy1339]

There's some funkyness going on with fetch-mock... It's complaining about "Unmatched GET"/ "No fallback response defined for GET to [..]"