Eslint, use src, update deps, use a utils module

[dgrammatiko]

This PR is:

• updating the dependencies to their latest versions
• dropping jshint in favour of eslint
• using the web-test-runner instead of the hihat
• move the source files in a src directory and
• splitting the source to essentially the class and a file named utils (this fixed Safari 🤷‍♂️)
• use both the default and a named export dragula

All tests are still green and the script works on all major browsers

If this is way too many changes for one PR I could split it in a few more specific ones.

BTW thanks for all the work you've already done here!

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