chore: add Svelte entry point to exports map

[metonym]

Closes https://github.com/metonym/svelte-highlight/issues/307

Add an explicit entry point for svelte in the package.json exports map.

Additional changes:

• Link to the hosted Changelog in the README (the Changelog will no longer be distributed with the published package)
• Use subpath exports syntax for styles/* and languages/*

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-308.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cluv3fnqd2ns73b868gg.