Cli: Consistent filename case

[ehmicky]

The CLI uses a kebab-case convention, while @netlify/build uses snake_case.
Should we keep it consistent between projects? If so, which one should we use?
I personally think snake_case is more idiomatic in Node.js, but I'm fine with any case, as long as we're consistent. What do you think @erezrokah?

Note: both currently lint this using the unicorn/filename-case ESLint rule, but with different options.

[erezrokah]

Let's go with snake_case

[erezrokah]

Going to do this once we clear most of the CLI pending PRs