Add support for relatedTarget

[ad-si]

relatedTarget is quite helpful for MouseEvents, FocusEvents, etc. where you are moving from one element to another one.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[garyb]

Thanks for the submission! But this already exists in the purescript-web-uievents library under the types it's specified for in particular:

• Web.UIEvent.FocusEvent
• Web.UIEvent.MouseEvent

[ad-si]

But there is no implementation for DragEvent. Am I supposed to convert to convert DragEvent -> Event -> MouseEvent? That feels wrong ...

[garyb]

Ah no, there definitely should be a total fromMouseEvent :: DragEvent -> MouseEvent conversion in https://github.com/purescript-web/purescript-web-html/blob/master/src/Web/HTML/Event/DragEvent.purs