Consider adding a rule for file naming

[jdforrester]

Our phpcs sibling has some strict rules around file naming (most notably, that files have only one class in them, and that the class name matches the file name exactly).

Do we want to do something similar? In general we seem to expect Title Case naming for files, though there's a mix of styles for how related test files are named (FooBarTest.js vs. FooBar.test.js vs. TestFooBar.js).