chore: memoize clsx() (alternative)

[adiguba]

Rewrite of the PR #15441

Call to clsx() are memoized in template_effect().
Same things for class directives, allowing to skip comparaison when they are not modified (as object is the same)

Exemple :

<div class={className} class:blue={blue}> </div>

Actual generated code :

	let classes;
	$.template_effect(() => {
		classes = $.set_class(div, 1, $.clsx($$props.className), null, classes, { blue: $$props.blue });
	});

With this PR :

	let classes;
	$.template_effect(
		($0, $1) => {
			classes = $.set_class(div, 1, $0, null, classes, $1);
		},
		[
			() => $.clsx($$props.className),
			() => ({ blue: $$props.blue })
		]
	);

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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:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
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Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 153875c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15456

[Rich-Harris]

nice!