pushChainContext() {
    this.chainContext = {
      upper: this.chainContext,
      countChoiceContexts: 0,
    };
  }
Push a new `ChainExpression` context to the stack.
This method is called on entering to each `ChainExpression` node.
This context is used to count forking in the optional chain then merge them on the exiting from the `ChainExpression` node.
@returns {void}