fix: prevent infinite loops when pruning CSS

[Rich-Harris]

fixes #14472, and also makes improves snippet resolution (it was only handling strings and identifiers, not literals, meaning answer={42} was interpreted as 'could be any snippet').

There might be other situations that could result in a loop, trying to think of any

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.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: a1330c8

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

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14474

[dummdidumm]

The "collect siblings/children" logic of :has is likely prone to this, too, as soon as you come across one render tag that iterates over all snippets (because of limited static analysis) and one of those snippets also has a render tag that references all snippets