Refactor and optimize parsers.js

[asamuzaK]

Summary

This PR refactors parsers.js to improve performance, maintainability, and code readability. The changes include optimizing lookup operations, eliminating redundant object creation in loops, reducing code duplication, and standardizing coding styles.

Changes

1. Performance Optimizations

• O(1) Lookups: Converted GLOBAL_KEYS (formerly GLOBAL_KEY) and SYS_COLORS (formerly SYS_COLOR) from Arrays to Sets. This changes the complexity of keyword checks from O(N) to O(1).
• Remove Dynamic RegExp: Replaced the dynamic creation of new RegExp inside loops within parsePropertyValue with string manipulation methods (slice). This reduces the overhead of compiling regular expressions on every iteration.

2. Refactoring & Maintainability

• Common Numeric Parser: Introduced a helper function parseNumericValue to centralize the parsing logic for numbers, lengths, percentages, and angles. This reduces code duplication across parseNumber, parseLength, parsePercentage, etc.
  • Design Note: The validation logic is passed as an anonymous callback function rather than being hardcoded in the helper. This design was chosen because the handling of unitless 0 varies across CSS types (e.g., allowed for <length>, but invalid for others such as <time>). Defining the condition explicitly in the caller function maintains clarity and flexibility for type-specific rules.
• AST Type Constants: Introduced AST_TYPES to manage AST node types (e.g., "Function", "Identifier") as constants, eliminating magic strings and reducing the risk of typos.
• Module Export Style: Refactored the module to define functions using const and export them via a single module.exports block at the end of the file.

3. Documentation

• JSDoc: Added comprehensive JSDoc comments to all exported functions to provide type information and improve the developer experience.

[domenic]

Looks great!

Just to confirm, this does not change any observable behavior or fix any bugs?