chore: refactor set_attributes code generation

[Rich-Harris]

This extracts the refactoring stuff out of #13338. It doesn't change any behaviour, it just dries the code out — at present there's a ton of duplication between RegularElement with a SpreadAttribute, and SvelteElement (which always uses set_attributes because we need to be conservative about custom elements).

The next step will be to generate code differently so as to fix #13270, but merging this first will reduce the noise of that work substantially. I'll self-merge this once green, since it's just moving some code around, so that I can continue working on that.

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