Add support for YAML configuration inheritance

[somiljain2006]

Fixes #1066

This change introduces _extends support in JCasC YAML configurations, allowing users to compose configurations across multiple files.

Features:

• Supports single and multiple _extends
• Resolves relative paths based on the current source
• Performs deep merge of mapping nodes
• Replaces sequences instead of merging them
• Detects circular dependencies in _extends chains
• Validates invalid usage (empty values, type mismatches)

Implementation:

• Added recursive resolution of _extends during YAML parsing
• Introduced caching to avoid re-parsing the same parent files
• Ensured immutability via node cloning during merge

Tests: Added unit tests covering inheritance, overrides, circular dependencies, invalid inputs, and edge cases

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[timja]

Is this needed when we have folders, other tools that can merge yaml e.g. kustomize, and yaml anchors?

[somiljain2006]

The goal of this is to provide a simple, native inheritance mechanism directly within JCasC without requiring external preprocessing or complex YAML constructs, unlike multiple files, which lack clear override semantics, or anchors, which are limited to a single file. This approach provides a very intuitive way to manage layered configurations.

[timja]

Yes but do people actually need it, there's a bunch of code there to maintain, limited interest in it recently on the issue.

I think there's probably more important issues to solve unless this is something you have a need for?

[somiljain2006]

I think it's better to convert this pr to a draft for now unless any meaningful use case appears for it.