Setup Jest for unit testing and add initial test

There are no Jest tests for this plugin, so it is needed to set up Jest and add the initial test.

• Install @folio/jest-config-stripes@^2.0.0 dev dependency

• Add jest.config.js in the root dir

• Add a script to run unit tests

• Add test/jest/_mock_ and test/jest/helpers directories

• Add initial test for some component