Fix str to snake to capture uppercase words followed by an underscore

[spicyPug02]

In ariadne_codegen/utils.py, str_to_snake_case was not capturing uppercase words followed by an underscore. \W in the regex would capture all non word characters, but not an underscore. So, things like 'Router_Get' and 'RouterBGP_Get' would both be converted to 'router_get' instead of 'router_bgp_get' in the second case. This caused issues in module naming in the generated clients. This commit fixes the issue by explicitly adding an underscore to the regex.

[rafalp]

Please rebase on main and add a short description of the change in CHANGELOG.MD

LGTM otherwise

[spicyPug02]

CHANGELOG.md is updated with a description of the fix

[rafalp]

Thank you for contributing!