fix: more efficient spread attributes in SSR output

[Rich-Harris]

Spread attributes are needlessly laborious in SSR output. Before:

import * as $ from "svelte/internal/server";

export default function App($$payload, $$props) {
	$.push();

	$$payload.out += `<div${$.spread_attributes(
		[
			{ "a": 1 },
			{ "b": 2 },
			{ "c": 3 },
			obj
		],
		true,
		true,
		""
	)}></div>`;

	$.pop();
}

After:

import * as $ from "svelte/internal/server";

export default function App($$payload, $$props) {
	$.push();
	$$payload.out += `<div${$.spread_attributes({ a: 1, b: 2, c: 3, ...obj })}></div>`;
	$.pop();
}

No test because there's no user-observable difference.

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Latest commit: 58336dc

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[Conduitry]

I see that something like <div a=foo {...spread1} b=bar {...spread2}></div> is now getting turned into

import * as $ from "svelte/internal/server";

export default function App($$payload, $$props) {
	$.push();
	$$payload.out += `<div${$.spread_attributes({ a: "foo", ...spread1, b: "bar", ...spread2 }, true, true, "")}></div>`;
	$.pop();
}

which seems reasonable.

But are there any other gotchas here? I'm not sure what they'd be. Do we know why the SSR code was written like that to begin with? Was it to mirror something in CSR (which turned out not to be necessary)?

[Rich-Harris]

Converting to draft for a moment as there's more we can do here.

I don't think there are any gotchas. I think we just fucked up before and made things needlessly complicated