Adding unit tests to modularize the visualizer

[adam2392]

Is your feature request related to a problem? Please describe.
Right now, the entire visualizer sits inside src/, which makes it difficult to unit test and parse.

Describe the solution you'd like
With unit tests added, we can add the following to the CI config:

- run:
          name: Jest tests
          command: yarn coverage --maxWorkers=2
      - run:
          name: Upload to codecov
          command: yarn codecov

Additional context
See https://github.com/bids-standard/bids-validator for an example of how they do some unit testing in javascript.