Missing snakecase, camelcase and kebabcase functions

[Ilyes512]

So I understand that we don't have those functions because they depends on a external package huandu/xstrings.

I was wondering if we could just manually add those functions by copy/pasting and of course cleaning up all the code we don't need from the xstrings package (with the copyright header intact of course) including the test file. See https://github.com/huandu/xstrings/blob/master/convert.go#L21

I am willing to make the PR, but wanted to check in first.

The functions at sprig: https://masterminds.github.io/sprig/strings.html#snakecase