Svelte 5 rewrite

Closes #12714

I initially did this change and then searched for where set_custom_element_data was used. I added the css_hash there too but then realized that for a normal custom element (rendered without the use of svelte:element it was already adding the class.

I did a bit of digging and found out that the css hash is added during the analysis phase which seemed a bit odd to me but seems part of a bigger system in place which i was a bit scared to touch). So for the moment i've made the css_hash argument optional and i'm checking for it's presence before adding the class but if you think a major refactor to always pass the css_hash during the transform phase instead of the analysis phase is needed i can work on this a bit more.

I've added the normal custom element to the test to avoid falling, in the future, in the trap of adding the custom css hash twice.

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
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• Ideally, include a test that fails without this PR but passes with it.

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