fix: use union type in DocumentPicker

[SimenB]

Why

This means that if type === 'success' then typeof uri === 'string' instead of string or undefined. This matches the docs as well: https://docs.expo.io/versions/latest/sdk/document-picker/#documentpickergetdocumentasyncoptions

On success returns a promise that resolves to an object containing { type: 'success', uri, name, size } where uri is a URI to the local document file, name is its original name and size is its size in bytes. If the user cancelled the document picking, the promise resolves to { type: 'cancel' }.

I suppose the undocumented fields in the return type is for web?

How

See "why". I made this change in the GitHub web UI.

Test Plan

I changed it locally and it works like I expected

[tsapeta]

Cool! Yeah, this previous type seems to be incorrect - thanks @SimenB for contributing! 🙇