Add SnakeCase Property Name Mapping Strategy

Description

This PR adds support for SnakeCase as a new property name mapping strategy in Mapperly, enabling automatic mapping between PascalCase properties in C# source objects and snake_case properties in target objects.

Motivation

In modern C# development, it's common to work with external systems or code generators that produce DTOs with snake_case property names (e.g., from JSON APIs, database schemas, or code generation tools). Currently, mapping between C# PascalCase properties and snake_case properties requires manual [MapProperty] attributes for each property, which is tedious and error-prone for large objects.

This feature is particularly valuable when:

• Working with auto-generated C# DTOs from external contracts that use snake_case naming
• Integrating with APIs that follow snake_case conventions (common in Python, Ruby, or REST APIs)
• The source contracts cannot be modified due to external dependencies or architectural constraints

Changes Made

• Added SnakeCase enum value to PropertyNameMappingStrategy with XML documentation
• Implemented snake_case to PascalCase conversion for property matching in MemberPathCandidateBuilder
• Updated MemberPathCandidateBuilder with new overload supporting naming strategies
• Modified MembersMappingBuilderContext to use SnakeCase strategy when configured
• Updated PublicAPI snapshot to reflect new enum value
• Added comprehensive test case for SnakeCase property mapping validation

Usage Example

[Mapper(PropertyNameMappingStrategy = PropertyNameMappingStrategy.SnakeCase)]
public partial class MyMapper
{
    public partial TargetDto Map(SourceDto source);
}

// Maps automatically:
// source.FirstName -> target.first_name
// source.LastName -> target.last_name

This allows developers to map properties between PascalCase source objects and snake_case target objects (e.g., FirstName -> first_name) by simply setting PropertyNameMappingStrategy = PropertyNameMappingStrategy.SnakeCase.

Fixes # (issue - if applicable)

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)