`emitCss: false` does not work with Svelte 5

[sissbruecker]

When using the plugin with the Svelte 5 compiler then emitCss: false is broken and the CSS source code is just stripped from the compiled code.

Reproduction:

• Clone this repo
• npm uninstall svelte
• npm install svelte@^5.0.0
• Add the following test and run npm test
• The test fails

test('injects CSS with `emitCss: false', async () => {
	const p = plugin({emitCss: false});

	const transformed = await p.transform(
		`
			<h1>Hello!</h1>

			<style>
				h1 {
					color: red;
				}
			</style>
		`,
		'test.svelte'
	);

	assert.ok(transformed.code.includes('color:red'));
});

[sissbruecker]

A workaround is to also specify the css option in the Svelte compiler options through the plugin config:

import svelte from "rollup-plugin-svelte";

svelte({
  emitCss: false,
  compilerOptions: {
    // Workaround for rollup-plugin-svelte not setting the compiler option when emitCss is false
    css: "injected",
  },
})