Convert project to an ES Module

[koddsson]

Since this is a big lift, I'm trying to split this work up into logical pieces.

This pull request sets the type key in package.json to module. I then mass renamed all the .js files to .cjs so we don't have to update all the require calls to import statements. By just doing this change, we can start to gradually convert CommonJS files (files with a .cjs extension) to ES module files (files with a .mjs or .js extension).

Closes https://github.com/chaijs/chai/pull/1411.