JavaScript: Add test suite

[MattIPv4]

We should add a test suite to the Javascript port of the package in this repo. It should test the behaviour of the exported generate method, but also the private methods involved in the generation process.

To test the private methods, it may be required that these are pulled out into a new file and exported/imported into the main lib file. They can then also be directly accessed from within the test suite.

The test suite for the main ruby package should act as inspiration for what should be tested, though the exact tests implemented are up to whoever takes this on -- https://github.com/MattIPv4/do_username/blob/master/ruby/spec/do_username_spec.rb