Add types declaration file, update objection to ^2.2.0

[geopic]

Resolves #17. As mentioned in the issue, updating objection to a more recent version is required to allow TypeScript support. It works fine from my own tests working from the code in the Readme but if there are any issues then please let me know.

[loteoo]

I manually used your PR in my project, but I made the types for the SDQueryBuilder class as such:

    import { Model, QueryBuilder } from 'objection';

    class SDQueryBuilder<M extends Model, R = M[]> extends QueryBuilder<M, R> {
        ArrayQueryBuilderType!: SDQueryBuilder<M>;

        NumberQueryBuilderType!: SDQueryBuilder<M, number>;

        delete(): this['NumberQueryBuilderType'];
        hardDelete(): this['NumberQueryBuilderType'];
        undelete(): this['NumberQueryBuilderType'];
        whereDeleted(): this['ArrayQueryBuilderType'];
        whereNotDeleted(): this['ArrayQueryBuilderType'];
    }

// And changed SDInstance to:

    interface SDInstance<T extends typeof Model> {
        QueryBuilderType: SDQueryBuilder<this>;
    }

[geopic]

Looks cleaner. If it works then all's good 👍

[loteoo]

Just updated the comment, we didn't need the SingleQueryBuilderType and the PageQueryBuilderType so I removed them. We would need them if the plugin added methods like .findById() and .page().

.delete(), .hardDelete() and undelete() internally use .patch(), which returns the NumberQueryBuilder.

.whereDeleted() and .whereNotDeleted() internally use .where() and .whereNot(), which returns the ArrayQueryBuilder.