[TypeScript] PipelineStage.Merge interface definition is wrong

[gabrie-allaigre]

Hi,

Introduced in Mongoose 6.1.0 PipelineStage.Merge has wrong definition:

export interface Merge {
      /** [`$merge` reference](https://docs.mongodb.com/manual/reference/operator/aggregation/merge/) */
      $merge: {
        into: string | { db: string; coll: string }
        on?: string | string[]
        let?: Record<string, any>
        whenMatched?: 'replace' | 'keepExisting' | 'merge' | 'fail' | 'pipeline'
        whenNotMatched?: 'insert' | 'discard' | 'fail'
      }
    }

'pipeline' is not a string, but a real pipeline stage array, https://docs.mongodb.com/manual/reference/operator/aggregation/merge/

And must be updated with:

export interface Merge {
      /** [`$merge` reference](https://docs.mongodb.com/manual/reference/operator/aggregation/merge/) */
      $merge: {
        into: string | { db: string; coll: string }
        on?: string | string[]
        let?: Record<string, any>
        whenMatched?: 'replace' | 'keepExisting' | 'merge' | 'fail' | Extract<PipelineStage, PipelineStage.AddFields | PipelineStage.Set | PipelineStage.Project | PipelineStage.Unset | PipelineStage.ReplaceRoot | PipelineStage.ReplaceWith>[]
        whenNotMatched?: 'insert' | 'discard' | 'fail'
      }
    }

Thanks