[12.x] Add XML conversion methods to Collection class and corresponding tests

[bipinks]

Add XML Conversion Support to Collection Class

Overview

This PR adds XML conversion capabilities to Laravel's Collection class, allowing collections to be easily converted to XML format with support for nested structures, custom element names, and proper character escaping.

Changes

• Added toXml() method to Collection class
• Implemented support for:
  • Custom root and item element names
  • Root element attributes
  • Nested collections and arrays
  • Special character escaping
  • UTF-8 and emoji support
  • Various data types (string, integer, float, boolean)
  • Empty and null values
• Added comprehensive test suite with 13 test cases

New Features

// Basic usage
$collection->toXml();

// Custom element names
$collection->toXml('custom_root', 'custom_item');

// With root attributes
$collection->toXml('root', 'item', ['version' => '1.0']);

// Nested collections
$collection->toXml();

Testing Notes

• Added 13 test cases covering:
  • Basic XML conversion
  • Custom element names
  • Root attributes
  • Nested collections
  • Special character escaping
  • Numeric keys
  • Empty collections
  • Null values
  • Large datasets
  • Data type preservation
  • UTF-8 characters
  • Maximum nesting level
  • Invalid XML element names
  • Special characters in attributes
  • Complex nested structures

Performance Considerations

• Implemented efficient XML generation using SimpleXMLElement
• Added size validation for large datasets
• Proper memory management for nested structures

Additional Notes

• Follows Laravel's coding standards
• Includes comprehensive PHPDoc comments
• Maintains backward compatibility
• No breaking changes
• Follows PSR-12 coding style

[taylorotwell]

I would suggest publishing this as a package that uses macros to add this to collection.