fix(demo): declare the sass dependency vite's scss compilation needs

[upupming]

demo/index.jsx imports style.scss, but sass was never declared in demo/package.json — only an orphaned node_modules/sass@1.53.0 entry in demo/package-lock.json kept it working:

• npm ci installs the lockfile verbatim (orphaned entries included), so the demo happens to run;
• a fresh npm install prunes the unreachable entry and rewrites the lockfile, after which npm run dev fails with:

[vite] Internal server error: Preprocessor dependency "sass-embedded" not found. Did you install it?
  Plugin: vite:css
  File: demo/style.scss

This reproduces on current main as well as the 10.x line (the orphaned entry dates back to the vite migration of the demo).

The fix declares sass@^1.53.0 — matching the version already present in the lockfile — so the lockfile change is a single line turning the orphaned entry into a reachable one. Verified that both npm ci and a fresh npm install now produce a demo where vite compiles style.scss successfully.

[coveralls]

coverage: 98.61%. remained the same — upupming:fix/demo-declare-sass into preactjs:main