Fix: CSS layer order in Next.js app

[nmn]

What changed / motivation ?

The nextJS example didn't have margins or paddings because the reset styles overriding all StyleX styles. (because of @CSS layers).

Fixed this by putting the reset styles in a CSS Layer themselves.

Additional Context

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 2.36 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/stylex.js	1.02 (3.53)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.34 (3.41)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1126.06 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	1005.35 (9859.49)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	120.70 (870.91)	0.00 (0.00)	0.0% (0.0%)