Add an export map

[Rich-Harris]

Is your feature request related to a problem? Please describe.

Svelte scores 88/100 on Skypack's new Package Score, because it doesn't have an export map (which explicitly states which files can be imported from an npm package. This is a relatively new thing that's supported by npm and some bundlers).

Describe the solution you'd like
Add an export map — should be very straightforward

How important is this feature to you?
I mean...

[Conduitry]

This should be all set now, but https://www.skypack.dev/view/svelte still isn't showing the latest version of the package and score. I don't know whether there's something we need to do to get them to re-calculate, or we just need to wait.