Svelte 5 rewrite

Fixes the problem delineated in #13654...the problem is when you render a snippet that render a component inside an element. This is because the code looks like this

App[$.FILENAME] = "App.svelte";

import * as $ from "svelte/internal/server";
import { createRawSnippet, mount } from 'svelte';
import Child from './Child.svelte';

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

	let count = 0;

	const hello = createRawSnippet((count) => ({
		render: () => `<div></div>`,
		setup(target) {
			mount(Child, { target });
		}
	}));

	$$payload.out += `<div>`;
	$.push_element($$payload, "div", 15, 0);
	hello($$payload);
	$$payload.out += `<!----></div>`;
	$.pop_element();
	$.pop();
}

App.render = function () {
	throw new Error("Component.render(...) is no longer valid in Svelte 5. See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information");
};

export default App;

specifically we are pushing_element before rendering the snippet and popping the element after. However inside the render in snippet we do

if (DEV) {
	// prevent parent/child element state being corrupted by a bad render
	reset_elements();
}

which set the parent to null...since this is happening within the div pop_element doesn't find the parent and errors out.

The original PR just safe guarded the parent.parent access which might be a solution nonetheless? My solution is storing the old parent and restoring it after rendering the component which seems a bit better but i'm not fully aware of the consequences.

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.

Before submitting the PR, please make sure you do the following

• 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.

Tests and linting

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