API improvements and refactoring.

[scalvert]

This PR changes a few things:

• Adds a new createProject callback in the options passed into createBinTester. This allows us to use either a constuctor, or some other form of factory function to create our Project instance
• Removes the projectConstructor option from the bin tester options. Users should use createProject to provide custom project implementations
• Changes the signature of runBin so that it takes rest args for the varargs passed to the bin, while still supporting the ability to use execaOptions on the end
• Renames .eslintrc.js and .prettierrc.js to .eslintrc.cjs and .prettierrc.cjs respectively
• Fixes BinTesterProject to correctly deep merge in writeJSON
• Cleans up the types so that they're more flexible for the new API for runBin
• Adds more tests