Add exports condition for Svelte in package.json

[mauroporras]

The package uses the legacy svelte field in package.json, but is missing an exports condition for Svelte. This causes a warning in vite-plugin-svelte 3+, and the fallback to the legacy field will be removed in a future major release.

The fix is to add an exports condition pointing to the same file as the svelte field:

"exports": {
  ".": {
    "svelte": "./dist/index.js"
  }
}

See: https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#missing-exports-condition