Don't break elements created before the polyfill

[KTibow]

Right now, if the polyfill is the second thing ran, it breaks all elements created before then with the error "Illegal constructor (custom element class must be registered with global customElements registry to be newable)". If you use bundlers like Rollup, Rollup may have just decided to move the polyfill down, breaking your code as a result. Not anymore.

This PR makes classes extended from raw HTMLElement initialize like they were extended from raw HTMLElement, fixing the issue. Would appreciate getting a release out soon, I'm building something with this polyfill.

*Maybe the polyfill should also do other things like relaying customElements.get to the native custom elements registry... not sure, that's for later discussion

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