chore(components): update @embroider/addon-dev v8

[aklkv]

📌 Summary

This PR updates the @hashicorp/design-system-components build to be compatible with
@embroider/addon-dev v8 by removing our reliance on rollup-plugin-scss and the
previous Rollup output behavior that only worked due to an Embroider bug.

What changed

• Replaced rollup-plugin-scss with a small Rollup “sidecar” plugin that:
  • Copies src/styles/** → dist/styles/** late in the build (during writeBundle)
  • Compiles our SCSS entrypoints to CSS using sass and writes them next to the
    published SCSS:
    • dist/styles/@hashicorp/design-system-components.css
    • dist/styles/@hashicorp/design-system-power-select-overrides.css
• Removed the SCSS entry from addon.publicEntrypoints() since styles are now published
  via explicit copying instead of Rollup’s module graph.
• Updated package.json exports to explicitly support existing consumer import paths:
  • @hashicorp/design-system-components.css
  • @hashicorp/design-system-power-select-overrides.{css,scss}
  • (optionally) deep style imports via ./styles/* mappings

Why it was needed

With @embroider/addon-dev v8, Rollup output handling is stricter and no longer allows
emitting files into arbitrary paths under dist. Our previous setup depended on a
known Embroider Rollup bug (embroider-build/embroider#2461), which caused the build to
break once upgraded.

By moving style copying and CSS compilation into a late build step and avoiding Rollup
asset emission for CSS entirely, the output becomes deterministic and fully compatible
with Embroider v8 while preserving the existing published structure and consumer-facing
import paths.

🛠️ Detailed description

📸 Screenshots

🔗 External links

Jira ticket: HDS-XXX
Figma file: [if it applies]

---

👀 Component checklist

• Percy was checked for any visual regression
• A changelog entry was added via Changesets if needed (see templates here)

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

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[didoo]

[blocker] we're moving away from this plugin, see https://github.com/hashicorp/design-system/pull/3237/files#diff-30635ef46c2e2d021e9e18afbf2cab0bfd4b5c3d8201db8c1321306396d28955

[aklkv]

I have put comprehensive explanation why it's needed I will let you decide which way you prefer it unblocks upgrade to for addon-dev and should go in soon to unblock gts migration work

[didoo]

In that case, please open also a PR on the main feature branch #3237 for Project Solar ("IBM carbonization" of HDS) so that your update works also on that branch, which represents a large (months) chunk of work; otherwise how would we "merge" the two approaches?

/cc @alex-ju @hashicorp/hds-engineering

[shleewhite]

Closing in favor of the approach taken in: #3509