Generated package.json files should include "types" field

[Rich-Harris]

Is your feature request related to a problem? Please describe.
When importing a library generated with svelte-kit package, types declared manually via .d.ts files aren't available idiomatically:

<script>
  import Circle from 'my-library/Circle.svelte';

  /** @type {import('my-library').Point} */
  const point = { x: 1, y: 2 };
</script>

<Circle center={point} radius={5}/>

Instead, you have to know where the types are defined inside the package — e.g. assuming you had a src/lib/types/index.d.ts file you would do this:

- /** @type {import('my-library').Point} */
+ /** @type {import('my-library/types/index.d.ts').Point} */
  const point = { x: 1, y: 2 };

This works because .d.ts files, like other files, get an entry in the generated "exports" map. But it's a bit ugly.

Describe the solution you'd like

• add a config.kit.package.types option, defaulting to "types" (or "."?), that gets included in the generated package.json
• when we get round to generating declaration files from components (and converting .ts files) in src/lib, we use this value as the output directory
• don't include .d.ts files in "exports"

Describe alternatives you've considered
Continuing to rely on "exports"

How important is this feature to you?
Important for authors and users of libraries created with svelte-kit package, but probably not urgent