Add missing export condition

[hi-brylle]

Whenever the svelte field is used in package.json, there needs to be an export condition in there as well. More info here: https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#missing-exports-condition.

I'm sorry if this a mess. I'm open for edits. This is my first time doing a PR but not my first time publishing to NPM so I only edited the package.json file in package directory, which also exactly contains the svelte field. The top-level package.json doesn't have the svelte field but it does have the exports field, but I'm not sure if I should touch it.