Unit tests

[doug-newman-nasa]

fixes #522
I've added the following in support of a unit test suite for STACBrowser:

1. Jest unit testing framework to package.json
2. Jest run script and coverage commands so that each 'npm test' will report and record code coverage with a desired goal of 80%
3. Provided an example unit test suite for Utils.js (the first two functions)
4. Minor update to README to document unit testing and fix the npm install command in Contributing section

[m-mohr]

Thanks @doug-newman-nasa.

As STAC Browser is primarily a (Vue-based) user interface, jest-based tests can only do so much. We can test some parts of the JS util files and some of the models, but most code can't directly be tested with the tooling that is introduced here (i.e. Jest).

Looking at the bigger picture, we need to extend this to also be able to test Vue components and UI interactions in general. Otherwise, the tests will not be very useful.

So we probably also need to introduce something like

• https://v1.test-utils.vuejs.org/guides/ (to test Vue components) and
• Karma or Puppeteer (to test UI interactions from a user perspective) - potentially there are also jest extensions for this?

There are more options out there it seems, like cypress, NightwatchJS, Playwright, Selenium, etc.

I don't have a lot of experince in any of the tools yet, but I think we should first check how this all can work together and whether Jest is a good choice together with the other testing libraries that we choose to use for Component and UI tests. If any of the other libraries already cover what Jest does, we may not even need Jest. As such I'm a bit hesistant to just merge this PR without looking at the bigger picture. We generally just need to ensure we make a good choice here as STAC Browser is quite a complex tool that will ultimately need a lot of tests, so making a bad choice here may waste a lot of time, which we don't have in open source ;-)

I don't have time to investigate the other parts yet, but hopefully have some time over Summer. If there's anyone who could help and has experience (or has time to gain experienence) and propose a solution, I'm happy to collaborate.