feat: add Query Builder whereExists methods

[memleakd]

Description

This PR proposes adding whereExists(), orWhereExists(), whereNotExists(), and orWhereNotExists() to the Query Builder.

EXISTS and NOT EXISTS conditions are common in real applications for filtering records by related data, checking membership/authorization relationships, and expressing “does a matching row exist?” without turning the query into a join.

$builder->whereExists(static function (BaseBuilder $builder) {
    $builder->select('1', false)
        ->from('orders')
        ->whereColumn('orders.user_id', 'users.id');
});

This keeps the condition inside the Query Builder flow instead of requiring users to write the EXISTS (...) clause manually. It also pairs naturally with the recently added whereColumn() support for correlated subqueries.

The API follows the existing Query Builder subquery style used by methods like whereIn(): it accepts a Closure or BaseBuilder instance. Raw SQL strings are intentionally not accepted; users who need fully manual SQL can still use where().

Changes

• Added whereExists() and orWhereExists().
• Added whereNotExists() and orWhereNotExists().
• Added user guide docs, class reference entries, and changelog entry.
• Added tests for Closure and builder subqueries, OR and NOT EXISTS variants, grouped conditions, bind handling, invalid inputs, and the existing same-builder guard.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide