Let's standardize window.orientation...

[marcoscaceres]

The WHATWG compat spec currently specifies window.orientation, but it could just be moved here. It's interoperable and I don't imagine it is going anywhere.

• move spec and update it
• add WPTs to check the orientation

[annevk]

That makes sense to me. It's always better to define features (even if they're bad) closer to the source.

[marcoscaceres]

ok, cool. I'll send a PR and coordinate with @miketaylr for the WHATWG compat side whatwg/compat#239