Fix types for ESM

[qurben]

Fixes GoogleChrome/dialog-polyfill#204

Should not have an impact on the cjs types.

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[qurben]

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[samthor]

So I confess I haven't been paying that much attention to the other issue. I appreciate that there is a problem, but I am confused. The dist esm file has this line in it:

export default dialogPolyfill;

That's literally the default export. It seems like this PR just adds another variable called dialogPolyfill of the core type. But we don't export that; we export a default.

[qurben]

I missed a line when applying my changes, sorry!

The created variable should be exported as the default. This tells Typescript that a value is exported an not a type.

[samthor]

Thanks. I'm learning about TS types vs variables too from this. I'll release an update soon.