Add support for Element.displayed

[tristanlabelle]

No description provided.

[jeffdav]

LGTM.

Can an element be present but not displayed? I'm assuming just expecting findElement to fail isn't a good solution (but we do that in notepad tests: https://github.com/thebrowsercompany/webdriver-swift/blob/main/Tests/WebDriverTests/NotepadTests.swift#L90).

[tristanlabelle]

@jeffdav I believe those test for different things. findElement should find anything that exists in the UI tree, displayed probably checks some visible flag or display: none in html. We'll see if this works for @stevenbrix 's scenario.

[tristanlabelle]

@jeffdav resetting your review now that this is in better shape :)

[stevenbrix]

@jeffdav yeah findElement still works when the element is in the tree but not visible on screen. excited to try this out @tristanlabelle !