Add JSON schema for PhpStan configuration validation

This PR introduces a schema.json file to validate the structure of the PhpStan configuration file (*.neon).

Motivation

• Improves developer experience by providing real-time validation and autocompletion in IDEs that support JSON schemas (e.g., PhpStorm, VSCode).
• Reduces configuration errors by catching typos, invalid keys, or incorrect value types early.
• Standardizes the expected configuration format across the project.

Changes

• Added schema.json in the phpstan directory.
• Schema covers all top-level keys: parameters and includes.
• Supports common PhpStan options like level, paths, excludePaths, ignoreErrors, reportUnmatchedIgnoredErrors, checkMissingIterableValueType, etc.
• Includes type definitions, default values, and descriptions for better clarity.

How to test

1. Link the schema to your PhpStan config file (e.g., via # yaml-language-server: $schema=https://raw.githubusercontent.com/fadrian06/phpstan/refs/heads/add-schema/schema.json).
2. Open the config file in a compatible IDE and verify that validation and autocompletion work as expected.
3. Intentionally introduce an invalid key or value to confirm that the schema catches the error.

Checklist

• Schema has been tested with a valid PhpStan configuration.
• Schema catches common misconfigurations.
• Documentation (if any) has been updated to reference the schema.