Improve return types for QueryBuilder methods

[earthiverse]

When I tried to chain select fields like

(new QueryBuilder('node'))
    ->selectField('Id')
    ->selectField((new QueryBuilder('Bearer_Token__c'))->selectField('value'))
    ->selectField((new QueryBuilder('Webhook_Endpoint__c'))->selectField('value'))

PHPStan was giving me errors, because selectField isn't accessible on AbstractQueryBuilder.

This commit fixes that.