perf: Optimize hot paths in utility functions

[Copilot]

Identified and eliminated performance bottlenecks in frequently-called utility functions during CSS processing.

Changes

String operations

• isColor(): Skip toLowerCase() when input contains no uppercase chars (most common case)
• convertUnderscoresToWhitespace(): Early return when no underscores present
• allVariablesAreUsed(): Replace RegExp creation in loop with string methods

Redundant checks

• expandSequence(): Remove duplicate numeric validation (already guaranteed by parent regex)
• hasMathFn(): Avoid template string creation, check all occurrences not just first

Data structure improvements

• Replace equality chains with Set.has() in isGenericName(), isAbsoluteSize(), isLineWidth(), isBackgroundPosition()

Example optimization:

// Before: 13 equality checks
function isGenericName(value: string): boolean {
  return value === 'serif' || value === 'sans-serif' || /* ... 11 more */
}

// After: O(1) Set lookup
const GENERIC_NAMES = new Set(['serif', 'sans-serif', /* ... */])
function isGenericName(value: string): boolean {
  return GENERIC_NAMES.has(value)
}

Impact: 11 functions optimized across 7 files. Reduces allocations and unnecessary work in paths called millions of times during builds.

Original prompt

Identify and suggest improvements to slow or inefficient code

[philipp-spiess]

(this is just me trying out githubs new agent features please ignore)