feat: Add vertical centering for operators and relations

[shubham030]

Summary

Adds vertical centering for binary operators and relations relative to numbers, improving visual alignment of math expressions.

Changes

New Features

• `centerOperators` flag in `MathOptions` (default: `false`) - when enabled, vertically centers operators (+, -, ×, ÷) and relations (=, <, >, ≤, ≥) at the math axis
• `forceVariableBaseline` flag for context-aware variable alignment:
  • Variables adjacent to numbers (like `3x`) stay at baseline
  • Standalone variables (like `x` in `3 + 5 = x`) get centered

Files Modified

• `lib/src/ast/options.dart` - Added new flags to MathOptions
• `lib/src/ast/syntax_tree.dart` - Context detection for variable-number adjacency
• `lib/src/render/layout/reset_dimension.dart` - Added verticalOffset parameter
• `lib/src/render/symbols/make_symbol.dart` - Alignment calculation logic
• `lib/src/render/utils/alignment_utils.dart` - New utility for offset calculations
• `lib/src/ast/nodes/symbol.dart` - Updated shouldRebuildWidget

Visual Effect

Before (default): Operators sit at baseline
After (opt-in): Operators vertically centered relative to numbers

Usage

```dart
// Default behavior (no centering, backward compatible)
Math.tex(r'3 + 9 = 12')

// Opt-in to centering
Math.tex(
r'3 + 9 = 12',
options: MathOptions(centerOperators: true),
)
```

Backward Compatibility

This change is fully backward compatible. The default behavior is unchanged - users must explicitly opt-in to operator centering.

[CLAassistant]

All committers have signed the CLA.