[offering pr] TypeScript Support

[wbhob]

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It is frustrating to have to build custom types to run alongside this library. I figure, if I'm going to do it anyway, I'd like to see if you'd accept a PR to add TS support.

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Write and bundle TypeScript types with this package for those of us using TypeScript.

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We were going to try to write our own types to use alongside this library to get strong typing. We're ending up having to import from 'sveltejs-forms/src/components/Form.svelte' to get type safety, and ideally we wouldn't rebuild from source if at all possible.

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[mdauner]

Great idea! Create a PR when you have it