Folder structure

[Rich-Harris]

This applies the changes discussed in #2790. I think it's a lot neater, and makes life easier in certain ways. For each sub-package (svelte/store, svelte/easing and so on) a stub package.json is created alongside a type declaration file.

The only possible danger here that I can see is that someone is using file extensions:

import { writable } from 'svelte/store.mjs'; // instead of 'svelte/store'

We could keep those files (re-exporting from the new ones, and printing a warning) but it feels like it's probably unnecessary. I can be persuaded if people feel strongly

[Conduitry]

This doesn't look like it does the stub package.json files thing. Was that intentional?

[Conduitry]

Update: prepublishOnly is what creates the stubs, and not build. This is bad and I am going to open a PR to change that.