Add unit and integration tests

[g-elwell]

This PR adds unit and integration tests using Jest.

Description

I've been playing around with this for a little while as a way of getting to grips with Jest, and feel like I've finally gotten to a point where it's worth sharing and discussing.

The solution I've landed on seems to work well. I've created some tests in the utils directory for each function in there, as well as a larger integration test in the root which is able to run various commands from the CLI and adjusts the mocked filesystem to emulate different outcomes.

To be perfectly honest I didn't realise there were tests running during CI already until I came to write this PR, but it feels like there's some value in this approach so hopefully it's beneficial.

Change log

• Adds unit and integration tests
• Adds jest and mock-fs as dev dependencies
• Adds npm script to run tests
• Updates use of package variable name to pkg to avoid issue with package being a reserved keyword which was flagged in Jest - docs

Sample test output

[g-elwell]

@ampersarnie do you have any objections or thoughts about these tests?

[ampersarnie]

@g-elwell Tests look great. Just have a few things which would be nice to have added relating to these;

1. An additional test script in package.json to run all tests once, and...
2. Have the tests running in the CI.

If you can add these as well, I'd be glad to approve them.

[ampersarnie]

Approving and addressing run command and ci seperately.