Add a simple test suite for cd commands

[astonm]

This PR adds a test suite for the cd command that I used to verify the correctness of #37. I pulled in jest as the test runner, but the same idea could be implemented easily with another option.

To run the tests, npm test.

As written, the codebase relies on a shared global namespace which happens naturally in the browser, but not-so-naturally when running tests from the command line, so I had to put in a hacky global.whatever = prelude to get things set up before the tests run. In order to get require to work, I added module.exports to the ends of the necessary files, and guarded those with a check so that when the same code runs in browser (with no module global) it won't complain.

Lemme know if you prefer a different set of hacks! I imagine without bringing in a build system, something like what's here is going to be one of the better options.

[astonm]

Just added some code to run the tests as part of the GitHub checks, but it's not enabled on this PR. I'll admit I have no clue how to make that happen... Happy to remove that commit if it's not needed.