feat: add isInstanceOfElement helper

[ph-fritsche]

Add a helper to verify the element type across libraries using testing-library/dom under the hood.

What:

Export a helper in @testing-library/dist/helpers to determine if an element is an instance of a specific HTMLElement type.

Why:

@testing-library/user-event requires a better way to determine the element type.
As the requirement is not specific to user-event, I think it would be the cleaner solution to add it here.

How:

The helper tries to find the element constructor on the window.

As the window is neither required to be associated with the observed document nor is it required to provide any element constructors, the helper provides a fallback for other environments than Jest+JSDOM.

Checklist:

• N/A Documentation added to the
  docs site
• Tests
• N/A? Typescript definitions updated
  There are no typescript definitions for internal methods at the moment.
• Ready to be merged

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[codecov]

Codecov Report

Merging #885 (79f8284) into master (5bc9364) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #885   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           26        26           
  Lines          951       959    +8     
  Branches       288       290    +2     
=========================================
+ Hits           951       959    +8     

Impacted Files	Coverage Δ
src/helpers.js	100.00% <100.00%> (ø)

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[eps1lon]

As the requirement is not specific to user-event, I think it would be the cleaner solution to add it here.

But only user-event needs this? Could you only implement this in user-event and then we'll see if we need it in other places as well and whether @testing-library/dom is the right place for this abstraction.

[ph-fritsche]

See testing-library/user-event#552

I've opened pull requests for both so we could implement it there and wait if this PR gets merged here.

As far as I know only user-event depends on identifying element types at the moment, but as the helper itself is strictly DOM related and has nothing to do with simulating events, collecting such abstractions here might prevent creating similar bugs in different places.

[eps1lon]

Thanks for the suggestions. We're going to let this incubate in user-event and see if and in what form we need it in /dom.

I suspect that most use-cases can live with a much simpler nodeType+localName check. That's what we're doing in dom-accessibility-api and we haven't heard any problems so far.