text: Title: feat: parser: walrus assignment operator without parentheses
  Body: I found that using walrus pretty useful with [decroator aliases](https://github.com/anki-code/xontrib-dalias) becuase it prints the result object immediately and the result stays in variable:
  ```xsh
  (r := $(@json echo '["hello", "world"]'))
  # ["hello", "world"]
  r[0]
  # 'hello'
  ```
  It will be cool to rid of brackets i.e. treat this example as python mode instead of subprocess:
  ```xsh
  r := 1
  # xonsh: subprocess mode: command not found: 'r'
  ```
  
  ## For community
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