Scope helper environment to only handle CSS

[markdalgleish]

This is a follow-up to #13008.

This PR makes the following refactors:

• Ensure the helper environment is only used for CSS. This is because the helper environment is Node-based and may differ from the ssr environment, e.g. when using vite-plugin-cloudflare. CSS is handled the same regardless of environment so it's safe to load from another environment, but we don't want to be loading arbitrary modules.
• In order to scope the helper environment to CSS, the server manifest is no longer loaded from the module graph during HMR. Instead, we now keep a reference to the current server manifest in memory.
• Hoists the CSS dev helper environment up to site alongside the other environments rather than being added on separately within the plugin.

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[markdalgleish]

The only consumer of this code threw an error if the result was undefined, so I've inlined that logic into this function now instead.

[github-actions]

🤖 Hello there,

We just published version 7.3.0 which includes this pull request. If you'd like to take it for a test run please try it out and let us know what you think!

Thanks!