Refactor `Command` and `Query` for DB 3.0

[Tigrov]

classDiagram
    Connection --> InsertCommand
    Connection --> SelectCommand
    Connection --> UpdateCommand
    Connection --> DeleteCommand
    
    InsertCommand --> InsertQuery
    SelectCommand --> SelectQuery
    UpdateCommand --> UpdateQuery
    DeleteCommand --> DeleteQuery

    InsertQuery -- InsertQueryBuilder
    SelectQuery -- SelectQueryBuilder
    UpdateQuery -- UpdateQueryBuilder
    DeleteQuery -- DeleteQueryBuilder

    class Connection {
        insert()
        select()
        update()
        delete()
    }

    class InsertCommand {
        execute()
        onConflict() / upsert()
        returning()
        batch()
    }

    class SelectCommand {
        all()
        one()
        column()
        scalar()
    }

    class UpdateCommand {
        execute()
    }

    class DeleteCommand {
        execute()
    }

Loading

1. The current Query needs to be split into SelectCommand and SelectQuery;
2. Everything related to Query should be moved from Command to SelectCommand;
3. SelectCommand contains SelectQuery and acts as a proxy;
4. Parts related to Query should be moved from QueryBuilder to SelectQueryBuilder and should work with SelectQuery;
5. SelectQuery must not depend on ConnectionInterface;
6. The remaining InsertCommand, UpdateCommand, DeleteCommand commands are done similarly, but with their own executing methods, for example InsertCommand::retuning() similar to the current Command::insertReturning();
7. InsertQuery, SelectQuery, UpdateQuery, DeleteQuery should follow to Remove methods replacing query condition #714
8. The corresponding methods for creating the commands should be added to ConnectionInterface;
9. Examples of how it will work after the changes:

$db->insert($table, $values)->execute();
$db->insert($table, $values)->onConflict($newValues)->returning(['id']);
$db->insert($table, $values)->batch();
$db->select($columns)->from($table)->where($condition)->all(); // as now
$db->update($table, $values, $condition)->execute();
$db->update($table, $values)->from($from)->where($condition)->execute();
$db->delete($table, $condition)->execute();

This will allow to add more options to commands and build more complex queries.