Allow underscore in variable naming convention

[0xalpharush]

It's pretty common to include underscores to prefix internal variables or to distinguish between arguments and state variables. We should not include these in the camel case naming convention detector

[stephenctw]

They claim to support the underscore rule but I also hit this issue.

[smallbatch-apps]

This is affecting us, too. The use of _name or name_ to prevent collision is very common, and necessary. Using _name should not trigger an error, as it is a standard convention.

[0xalpharush]

This lines need to be update to include internal state variables: https://github.com/crytic/slither/blob/0ec487460690482c72cacdea6705e2c51bb3981e/slither/detectors/naming_convention/naming_convention.py#L165

[devtooligan]

bump