Readonly property permission is missing in Notification global variable

[IlyaVassyutovich]

Hello.

Issue title pretty much describes the problem. Readonly property permission with status of permission for web notifications is not emitted in global variable, but present in interface declaration.

Unfortunately, I don't know F# at all... I've tried to fix this using addedTypes.json, but when I add this entry, readonly property permission gets emitted in every single one interface and global variable declaration. And in the docs there is note stating that modification of browser.webidl.xml is forbidden, because it's generated by Microsoft Edge.

{
	"kind": "constant",
	"name": "permission",
	"type": "NotificationPermission"
}

I can make a PR showing my problem, should I?

[mmermerkaya]

Related issue: microsoft/TypeScript#14701