Support for extending native elements via polyfill

[semmel]

As long as Safari refuses to extend native elements like HTMLButtonElement a polyfill is needed for that. I've tested @ungap/custom-elements-builtin recently with Safari 13.1 and Safari TP 109.

This PR

• makes osagai compatible with @ungap/custom-elements-builtin (See test/define.test.js)
• migrates the define() tests to Karma for testing on real browsers without mocking the CustomElements API
• adds a test for extending native elements
• removes all the babel dependencies which came with Jest
• removes rollup plugins without function

Concerning the tests:

• The other tests need to follow, but that maybe a future effort.
• I used chai for assertion helpers - sorry but it was what I am familiar with.
• Test run fine locally, however
• Concerning Travis CI tests I don't know why it reports they have no browsers installed:

Cannot start Firefox 239 Error: no DISPLAY environment variable specified
No binary for Safari browser on your platform.
No binary for ChromeCanary browser on your platform

If that is the case, such a CI looks pretty useless to me...

There are a lot of changes in this PR and I did not bother to match the projects code style. Right now that would have been too much of a burden to me and my time is scarce.
I like to present my work here anyway because I think the added functionality is useful and this project has been silence for a while now.