Add LRU cache for CSS parsing and validation

[asamuzaK]

Summary
Adds an LRU cache to reduce repeated css-tree parsing/validation and improve performance of CSS parsing utilities.

Part of jsdom/jsdom#3985

What this PR does

• Adds lib/utils/cache.js: simple LRU wrapper using lru-cache (max: 4096) with a null sentinel for null values.
• Caches results in lib/parsers.js for:
  • isValidPropertyValue (key: isValidPropertyValue_${prop}_${val}) — caches boolean results
  • resolveCalc (key: resolveCalc_${val}) — caches resolved string value
  • parsePropertyValue (key: parsePropertyValue_${prop}_${val}_${caseSensitive}) — caches parsed AST/plain-object results or false when invalid
• Adds lru-cache to package.json and package-lock.json.

Benchmarks
cssstyle main:

> jsdom@27.4.0 benchmark
> node ./benchmark/runner --suites style

# style/createElement + setAttribute('style') #
jsdom  x 44.33 ops/sec ±1.27% (59 runs sampled)

# style/createElement + style properties #
jsdom  x 142 ops/sec ±3.34% (72 runs sampled)

# style/createElement + style.cssText #
jsdom  x 36.25 ops/sec ±2.77% (59 runs sampled)

# style/innerHTML: divs with inline styles #
jsdom  x 41.36 ops/sec ±2.46% (55 runs sampled)

# style/innerHTML: simple divs (no styles) #
jsdom  x 3,739 ops/sec ±1.71% (88 runs sampled)

cssstyle PR:

> jsdom@27.4.0 benchmark
> node ./benchmark/runner --suites style

# style/createElement + setAttribute('style') #
jsdom  x 178 ops/sec ±1.46% (83 runs sampled)

# style/createElement + style properties #
jsdom  x 248 ops/sec ±3.14% (78 runs sampled)

# style/createElement + style.cssText #
jsdom  x 148 ops/sec ±3.85% (76 runs sampled)

# style/innerHTML: divs with inline styles #
jsdom  x 154 ops/sec ±2.61% (79 runs sampled)

# style/innerHTML: simple divs (no styles) #
jsdom  x 3,837 ops/sec ±2.53% (89 runs sampled)