Fix Value<D> and export getExportURL function type

[axelboc]

Partial fix for #1063

There was a problem with the Value<D> type. In advanced cases, like the one identified by @bmaranville, the shape and dtype of the Dataset generic was not properly inferred.

getExportURL is only called with array-shaped datasets (Dataset<ArrayShape>), but Value<D> was not being narrowed down at all. Now, Value<D> is properly narrowed down to unknown[] | TypedArray (if we go down to the barest types).

getExportURL={(format, dataset, selection, value) => async () => {
  value.forEach((val) => { /* ... */ }); // `forEach` now works because it is available on both arrays and typed arrays
}}

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I'm also exporting the type of the getExportURL function in @h5web/app so one can extract the function outside of the JSX:

const getExportURL: GetExportURL = (format, dataset, selection, value) => { ... };

I'm usually not a fan of function expressions, but in this case, replicating the signature would require exporting a lot of types and would be prone to mistakes. Indeed, with a function declaration, given the permissive nature of TS, consumers may not end up typing the function as strictly as they could and therefore struggle to implement it.