Rename atom.syntax to atom.grammars

Previously, `atom.syntax` was public but contained no public methods. I was considering removing it entirely when I noticed that we use `atom.syntax.selectGrammar` in some package specs, so I decided to make it officially public. The name `syntax` feels pretty vague now, and in the spirit of focusing our global objects on specific tasks, I think the name `grammars` blends in better with the rest of our global services.