Add Ruff rule: invalid-function-name (N802)

[genedan]

This rule enforces PEP 8 function naming conventions (snake_case). i.e.,

def myFunction():
    pass

Should be renamed

def my_function():
    pass

Functions flagged in the test suite can be safely renamed. Functions in the codebase would need to go through the deprecation process (or excluded from the rule if there's a good reason, i.e., API compatibility with a mirrored package).