feat: add load function to handle hook to control css loading

[sviripa]

This PR introduces an additional handle hook function, similar to preload. The purpose of this function is to control which CSS assets should be loaded for a specific page or route.

This functionality is necessary for dynamic component use case implemented using a combination of +page.server.js and +page.js. In the current version of SvelteKit, the CSS assets of all dynamically imported components are loaded on the page, regardless of whether or not they are actually used.

It is possible to remove unwanted css by redacting the html output using transformPageChunk but this feels more like a hack. It also does not work with inlineStyleThreshold configuration property.

The new function will allow SvelteKit users who use dynamic component imports to control which CSS assets should be loaded.

I'll address the items in the list below once I receive initial feedback on the solution. Thanks 🙏🏻

TODO:

• Naming
• Test
• Docs
• Changeset

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Tests

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P.S.

Huge thank you to everyone involved in Svelte and SvelteKit projects ❤️

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Latest commit: 9230536

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