LibWeb: rename 'cross-origin opener policy' to 'opener policy'

[0xkon1]

Closes #1387

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[AtkinsSJ]

Thanks for doing this!

I've marked a few places where your changes don't quite match the spec text. We're quite fussy about that. :^)

Also in two places the spec still says "cross-origin opener policy". That seems to be a mistake in the spec so while I've noted them, you can ignore them. I've opened whatwg/html#10631 to fix the spec.

When making corrections, please make the changes to your existing commit. This video is good at explaining how to do that in case you're not super familiar with git. You can also ask here or on Discord if you get confused.

[0xkon1]

Thank you for the feedback. I could have done it easier by just copying the spec text. Should al be corrected now.