message: fix: update type declaration for CstChildrenDictionary
  When an optional element is absent in a node, Chevrotain sets that element to `undefined`.
  CstChildrenDictionary has been updated to allow for children to be `undefined`, in order to reflect this.
  Fixes
diff: @@ -1964,7 +1964,7 @@ export interface CstNodeLocation {
  }
  export declare type CstChildrenDictionary = {
  - [identifier: string]: CstElement[]
  + [identifier: string]: CstElement[] | undefined
  }
  export declare type CstElement = IToken | CstNode
  