Refactor, put all logic in vanilla JS files

[loket]

It's a bit untidy to have all the logic in .njk files, and need to run eleventy in order to perform tests. It would be better if all the logic would be put in, say, app.js, and the generated output could be put in, say app.bundle.js (which could contain all the words).

This would also allow for testing with i.e. Jest (by adding a app.test.js file), which doesn't require a bunch words from another file to work.

[loket]

Should consider inlining everything into one file.