Separate the svelte component and the js wrapper

It would be nice to bring all the functionality that is needed for use without svelte into a separate component that will be imported from js.
And the pure svelte component will be imported from svelte.
You also need to make native events in the svelte component (and pass them above), and use callbacks in the wrapper, etc.

The issue names no files, tests, or entry points. First map the existing Svelte component and JavaScript wrapper, then define the shared functionality and event/callback boundaries; done means separate non-Svelte and pure Svelte imports with native events forwarded and callbacks used by the wrapper.

Written by the indexing model from the issue text.