chore: refactor RegularElement visitor

[Rich-Harris]

This is a precursor to #13270. We need to change how we deal with the combination of attributes and class/style directives, and ideally unify how attributes are handled between RegularElement and SvelteElement. A first step towards that is moving the special case logic (for specific elements like <img> or <input> or whatever) out of the loop in which we separate attributes out into their different types.

This PR just tries to make the logic a bit less entangled, and a bit easier to follow, with fewer variables declared far from their usage sites etc.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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