Limited tree-shaking when Temporal is imported but not used

[federicocarboni]

Bundlers are not able to remove the temporal polyfill when it is imported but not used. This produces large bundles even when Temporal functionality is not used.

import {Temporal} from "@js-temporal/polyfill";

export function usesTemporal() {
  // Anything with Temporal
  return Temporal.PlainDateTime.from("1970-01-01");
}

export function notUsingTemporal() {
  return 42;
}

Now even if the user of the library only imports notUsingTemporal a large amount of code from the temporal polyfill is included anyway.

This is very important for library authors who want to use Temporal but allow it to be tree-shaken out if not necessary.

[federicocarboni]

Fixed in #287