fix: load and error dispatch `Event` instead of `UIEvent`

[eps1lon]

What:

fireEvent.load (et. al.) dispatches a Event now instead of UIEvent Adds a test for https://github.com/testing-library/dom-testing-library/issues/498

Why:

I imagine that the most common targets are <img /> and <script /> and neither of these receive load events as UIEvent: https://codesandbox.io/s/react-script-does-not-work-the-way-you-think-it-does-stc66c

How:

Changed event type and added tests for <script />. Ieft a note that our abstraction doesn't support firing events with different types.

Also removed the notion of "Image events" from comments. We refer to DOM interfaces in all other cases. But there is no such thing as ImageEvent.

Checklist:

• ~[ ]~ Documentation added to the docs site
• [x] Tests
• ~[ ]~ TypeScript definitions updated
• [x] Ready to be merged

[codesandbox-ci[bot]]

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