test: remove deprecation warnings

[bluwy]

Description

• Use "type": "module" to remove Vite's CJS deprecation message
• Update postcss.config.js to handle ESM/CJS syntax
• Update scss to @use instead if @import as its deprecated (logs a warning in vite 6)

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/@vitejs/plugin-vue@466

pnpm add https://pkg.pr.new/@vitejs/plugin-vue-jsx@466

commit: aa81b84