Add Window and Window Management API

[autonome]

A couple notes:

• I moved alert, prompt and confirm to the <dialog> feature, which is fiiine, but doesn't feel right. The developer pattern seems to be script-first, not element-first wrt to these features, so rolling these APIs up under <dialog> isn't ideal if looking at through Developer Vision. Could separate into two features or leave as-is for now and break it down later.
• A number of keys under window are promised to other features already, which I've noted in comments for now. If this PR gets a 👍🏼 I'll remove those comments before merging, but wanted them there for visibility during review, vs looking like a bunch of keys are randomly missing.

[autonome]

# baseline: false
# support:
#   chrome: "1"
#   chrome_android: "18"
#   edge: "12"
#   firefox: "1"
#   firefox_android: "4"
#   safari: "1"
- api.Window.open

hmm need to check this out

update: asked @Elchi3 about this. the BCD seems right, but Safari iOS not showing up here.

[Elchi3]

BCD has partial_implementation for window.open https://github.com/mdn/browser-compat-data/blob/85134660ebd65ba1ff2d2561f24fa81ddbbc6619/api/Window.json#L3378

[autonome]

@ddbeck I'm reluctant to call this not baseline high for the window.open issue. That API has been around since the dawn of the web. Outside of that issue, should be ready for review here.

[ddbeck]

A few suggestions for you. This is pretty close though.

[autonome]

Thanks for review, fixes applied!