Implement optional right-to-left mode with arrow syntax

[konard]

Summary

This PR implements optional right-to-left mode with arrow syntax support as requested in issue #63.

Features Added

• Arrow Syntax Support: New support for directional arrow operators (← and →) in both single-line and multiline formats
• Expression Equivalence: Implements the equivalence mentioned in the issue:
  • (2 ← 1) = (1 → 2) = (2 1)
• Flexible Formatting: Links can now be formatted with arrow syntax using the new arrowMode parameter
• Comprehensive Grammar: Updated PEG.js grammar with proper precedence to handle arrow operators

Technical Implementation

1. Grammar Updates (grammar.pegjs):

   • Added arrowLink and multiLineArrowLink rules
   • Excluded arrow characters from referenceSymbol to prevent conflicts
   • Proper transformation of directional expressions

2. Parser Enhancement:

   • Arrow expressions are parsed into standard link structures
   • 2 ← 1 transforms to { values: [1, 2] } (right points to left)
   • 1 → 2 transforms to { values: [1, 2] } (left points to right)

3. Link Class Extensions:

   • Added arrowMode parameter to format() method
   • New arrow formatting for 2-element links: link.format(false, false, true) → "1 → 2"
   • Backward compatible with existing formatting modes

4. Test Coverage:

   • Comprehensive test suite covering all arrow syntax scenarios
   • Tests for parsing equivalence between different formats
   • Tests for arrow formatting mode
   • Tests for complex expressions with quoted references

Examples

import { Parser, Link } from '@linksplatform/protocols-lino';

const parser = new Parser();

// All of these produce the same result: { values: [Link(1), Link(2)] }
parser.parse('1 → 2');       // Left arrow
parser.parse('2 ← 1');       // Right arrow  
parser.parse('(1 → 2)');     // Multiline left arrow
parser.parse('(2 1)');       // Standard format

// Arrow formatting
const link = new Link(null, [new Link('1'), new Link('2')]);
link.format(false, false, true);  // Returns: "1 → 2"

Test Results

All tests pass including:

• 8 new arrow syntax tests
• All existing 80+ tests continue to pass
• No breaking changes to existing functionality

Fixes #63

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