* Make the nodes fields filtering process - from libcst.tool - public, so that other libraries may provide their own custom representation of LibCST graphs.
* Create functions to access & filter CST-node fields (with appropriate docstrings & tests), in libcst.helpers
* Add new CST-node fields functions to helpers documentation.
which ensures we won't have inconsistent black-vs-isort errors
going forward. We can always format by running `ufmt format .`
at the root, and check with `ufmt check .` in our CI actions.
