fix: detect Deno workspace root (fix #22237)

[semimikoh]

Description

Adds Deno workspace detection to searchForWorkspaceRoot.

Deno workspaces are declared with the workspace field in deno.json. Without recognizing this config, Vite may resolve the workspace
root to a nested package directory instead of the repository root. This can make the default server.fs.allow too narrow for Deno
projects where npm packages are installed under the repository-level node_modules/.deno directory.

This became visible after server.fs checks started applying to environment transport module fetches.

Changes

• Detect deno.json and deno.jsonc files with a workspace field in searchForWorkspaceRoot
• Add Deno workspace fixtures
• Add tests for nested and root Deno workspace resolution

Notes

deno.jsonc is detected only when its contents are also valid JSON. Full JSONC parsing would require an additional parser, which this PR
avoids.

Tests

pnpm exec vitest run packages/vite/src/node/server/tests/search-root.spec.ts

Fixes #22237

[bluwy]

I think it might be worth looking into supporting deno.json in searchForPackageRoot too, but I think this change itself is fine to fix the issue first now.