Typings: Add knex() method type to QueryBuilder

[pigpudle]

Objection.js allows us to get Knex instance from QueryBuilder by using .knex() method like this:

await ModelName.query(trx).knex().raw(query);

Source code: https://github.com/Vincit/objection.js/blob/master/lib/queryBuilder/QueryBuilderOperationSupport.js#L194

But this method is not defined in types definitions. Thus TypeScript shows an error like Property 'knex' does not exist on type 'QueryBuilder<ModelName, ModelName[]>' when using the above code.

This PR adds .knex() method definition to QueryBuilder types.

[lehni]

Looks good, thank you @pigpudle!