text: Title: Feature: Transformed captured output `$func()`
  Body: After merging #5377 I remember about old proposal. I'm not remember the author but It's interesting idea.
  
  It will be cool to have an ability to create transformer of captured output:
  ```xsh
  json = $json(curl https://myjsonapi)  # python mode, returns json object
  
  def mytransformer(stdout):
      # my transformations
      return stdout
  
  my = $mytransformer(echo 123)  # python mode
  
  echo $mytransformer(echo 123) | grep 1  # subprocess mode
  ```
  
  ## For community
  ⬇️  **Please click the 👍 reaction instead of leaving a `+1` or 👍  comment**
  