feat: add directional gap utilities

[e-simpson]

Summary

This PR adds Tailwind-compatible directional gap utilities for React Native flex and grid layouts.

Changes

Core Implementation

• Update: src/parser/spacing.ts
  • gap-x-{value} maps to React Native columnGap
  • gap-y-{value} maps to React Native rowGap
  • Reuses the existing spacing scale, including fractional and px values
  • Supports arbitrary pixel values
  • Supports custom values from theme.extend.spacing
  • Preserves existing gap-{value} behavior

Testing

• Update: src/parser/spacing.test.ts
• All 77 focused spacing tests pass
• Covers preset, fractional, arbitrary, zero, px, and custom-theme directional gaps

Documentation

• Update: docs/src/content/docs/reference/spacing.md
• Added directional and arbitrary-value examples

Compile-Time Transformation

// Input
<View className="gap-x-2 gap-y-1.5" />

// Output
<View style={{ columnGap: 8, rowGap: 6 }} />

Validation

• bun run vitest --run src/parser/spacing.test.ts (77 tests)
• bun run check
• bun run lint
• bun run format:check
• bun run build