Feature: Transformed captured output `$func()`

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:

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

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