vanilla-javascript

Convert the source file to typescript, which will be compiled in the rollup build step.
While we currently use .ts files, typehinting etc. is not used in the files.

If you want to help:
Just start adding typehints to any file that is still missing them or improve the ones that already have them. Feel free to send a PR for files even if only a part of them is converted.

Is your feature request related to a problem? Please describe.
Tests are an important part of trusting that a bunch of code will work as intended and ensure not to break a project when updating it. It would be great to add/update tests for Darken.

Describe the solution you'd like
Update and add missing tests using Jest (already installed in deps).