GSOC2025: Json Library for D, weekly update #1

[Gaofei]

So far, the main focus has been on refactoring the 
deserialization logic using a policy-based design , which allows 
users to customize the deserialization process (e.g., depth 
limits, field validation, handling unknown fields, etc.). A basic 
framework for policy-based deserialization has already been 
built, including:

A DefaultDeserializationPolicy with support for:
Maximum depth control
Optional field handling
Field visit tracking
Error reporting for unknown fields

Policy hooks:
onObjectBegin
onField
onObjectEnd

All stuff above are in this PR: 
https://github.com/schveiguy/jsoniopipe/pull/36