Add helper functions for common ways of filtering nodes

[zaicruvoir1rominet]

Summary

Currently, the way to print out the CST is using libcst.tool.dump.
There are many useful inner functions in libcst.tool which could be used by 3rd parties to provide their own custom representation of LibCST graphs.

The goal of this PR is to expose some of these functions: functions which help to filter all the non-relevant data from the CST.

Changes

Following functions have been created in libcst.helpers:

• get_node_fields
• is_whitespace_node_field
• is_syntax_node_field
• get_field_default_value
• is_default_node_field
• filter_node_fields

And libcst.tool.dump now uses filter_node_fields.

Test Plan

Parse a simple CST, then check CST-nodes have their fields properly filtered when using filter functions.

[zsol]

High level making these public makes sense, but:

1. it's worth extracting _is_syntax and _is_whitespace into their own public functions (with docstrings and tests)
2. move all of these under libcst.helpers, please

[zaicruvoir1rominet]

Following functions - with docstrings and tests - have been created in libcst.helpers:

• get_node_fields
• is_whitespace_node_field
• is_syntax_node_field
• get_field_default_value
• is_default_node_field
• filter_node_fields

All above functions are now part the Sphinx Helpers doc.

libcst.tool.dump now uses filter_node_fields.

[zaicruvoir1rominet]

(Please check the tests ; I was not really sure how to properly cover use cases, so do tell if work needs to be done there)

[codecov]

Codecov Report

Attention: Patch coverage is 97.98658% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.15%. Comparing base (9f54920) to head (fd3646b).
Report is 9 commits behind head on main.

Files	Patch %	Lines
libcst/helpers/node_fields.py	93.75%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1137      +/-   ##
==========================================
+ Coverage   91.12%   91.15%   +0.02%     
==========================================
  Files         256      258       +2     
  Lines       26567    26689     +122     
==========================================
+ Hits        24210    24328     +118     
- Misses       2357     2361       +4     

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[zsol]

Gotta fix the type errors and this is good to go.

[zsol]

Thank you!