Camel-case: Add '_' to a separator list

[seclerp]

Currently, to_camelcase takes into account only '-' as a separator (because of CSS' kebab-case). But there are also a lot of places where _ is used (snake_case), it leads to inconsistent naming issues.

Please note that this PR introduces breaking changes to existing names.

[juliettef]

Good point. I think the original implementer's reason for removing the - is because it's not permitted in variable names in C#.

At this point I'd rather keep them as they are to avoid introducing a breaking change. What do you think?