Add support for nested meta queries

[johnbillion]

Fixes #45

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[Copilot]

Pull request overview

This PR adds support for nested meta, tax, and date queries by allowing Query objects to contain other Query objects in addition to Clause objects. This mirrors WordPress core's native nested query functionality.

Key changes:

• Added addQuery() methods and $queries properties to all three query types (MetaQuery, TaxQuery, DateQuery)
• Implemented isFirstOrderClause() methods to distinguish between clauses and nested queries during deserialization
• Updated toArray() methods to serialize both clauses and nested queries with proper index handling

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/MetaQuery/Query.php	Added nested query support with addQuery() method, $queries property, and isFirstOrderClause() logic
src/TaxQuery/Query.php	Added nested query support with addQuery() method, $queries property, and isFirstOrderClause() logic
src/DateQuery/Query.php	Added nested query support with addQuery() method, $queries property, TIME_KEYS constant, and isFirstOrderClause() logic
tests/phpunit/MetaQueryTest.php	Added three new test cases for nested queries and updated test method name for clarity
tests/phpunit/TaxQueryTest.php	Added three new test cases for nested queries (associative, indexed, and manual construction)
tests/phpunit/DateQueryTest.php	Added three new test cases for nested queries (associative, indexed, and manual construction)
tests/WP_Query.php	Added example usage of nested meta queries
README.md	Added documentation example demonstrating nested query usage
composer.json	Reordered test script to run phpunit before phpstan

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