batchInsert fix return type in typings

[maximelkin]

Environment

Knex version: latest
Database + version: any
OS: any

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Bug

1. Explain what kind of behaviour you are getting and how you think it should do
   knex.batchInsert as for documentation returns knex.QueryBuilder
   actually it is Promise & {returning, transacting}

[kibertoad]

Released in 0.21.13