Add option to hide __ members from auto complete.

[karthiknadig]

For microsoft/vscode-python#17776

[pappasam]

@karthiknadig does this feature request only apply to class attributes / methods that begin with __ but that do not end with __? For example, __init__ technically begins with __ but the __init__ method is different from something like __private_attribute. See https://docs.python.org/3/tutorial/classes.html#private-variables.

This leads me to another design question: do we want to support removing either private variable completion (single underscore start followed by non-underscore) and mangled private variable completion (double underscore start, at most single underscore finish)?

[karthiknadig]

The feature request applies to all variables that have __something and __init__ patterns.

If we have levels of filtering or regex patterns, I think, that would work better, this way we can control the behavior of flags like python.autoComplete.showAdvancedMembers from the IDE. So, if we want to change the meaning of true to include since-underscore variables.