Feature: Support for loading components by promise

Hi, if you do like this

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It won't load the component and will give error like; `component.then is not a function`
Maybe you can detect is it a promise in loader and if it is, it can await it to load?

Start at the Loadable component's loader handling and reproduce the issue with the provided dynamic-import example. Trace why the loader value does not expose then, and consider the work complete when a promise-based loader successfully renders Test.svelte without the component.then error.

Written by the indexing model from the issue text.