fix(exports): include additional `exports` in package.json

[metonym]

Fixes #315

package.json#exports needs additional entries to the library code.

This fixes resolution of the following import methods:

// Base import for styles/languages
import { _default } from "svelte-highlight/styles";
import { _1c } from "svelte-highlight/languages";

// Explicit index
import { _default } from "svelte-highlight/styles/index";
import { typescript } from "svelte-highlight/languages/index";

// Explicit extension
import lang from "svelte-highlight/languages/1c.js"
import style from "svelte-highlight/styles/3024.js";