fix: css sourcemap generation with unicode filenames

[gtm-nayan]

fixes #6714

code-red will fail on such cases too, but that codepath doesn't seem to be hit by Svelte currently, if the error pops up again that'd be the place to look

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 4edc308

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[gtm-nayan]

/ecosystem-ci run

[svelte-ecoystem-ci]

📝 Ran ecosystem CI: Open

suite	result
eslint-plugin-svelte	✅ success
language-tools	✅ success
mdsvex	✅ success
prettier-plugin-svelte	✅ success
rollup-plugin-svelte	✅ success
skeleton	✅ success
svelte-eslint-parser	❌ failure
svelte-loader	✅ success
svelte-preprocess	✅ success
sveltekit	✅ success
vite-plugin-svelte	✅ success

[gtm-nayan]

eslint parser has been failing on master, should be fine