Ignore "free functions" when checking for visibility.

[kaspergff]

When creating a free function (a function outside of a contract) I get the following error: Linter: Explicitly mark visibility in function [func-visibility

But as explained here

"Free functions are always internal functions and are meant to replace internal library functions and their very special behaviour."

. Therefore they don't need a visibility keyword.

[PaulRBerg]

Can confirm that this is still occurring on the latest version (v3.3.7).

It's a shame because I would like not to disable the func-visibility rule.