Turn capitalization conventions into language rules for 1.0

[Pretty][Pretty]

[Pretty]: https://github.com/glaebhoerl/rfcs/blob/significant-identifier-capitalization/active/0000-significant-identifier-capitalization.md
But there are other conventions which could also prevent such mistakes, right? So a lint seems appropriate since we are guiding the user to a particular convention, but as long as they follow _some_ convention, they will be OK.