[w3c/permissions] Add "popups" permission type (#210)

I have a use case where I'd like to open two popups at a time when acting upon a user gesture. In Chrome, this results in one of the popups getting blocked unless the user has set the permission to allow popups from my website to open.

![image](https://user-images.githubusercontent.com/1656324/85962718-2cfdbb80-b980-11ea-9308-2fb00885def8.png)

It'd be great if I could prompt the user to allow popup permission when one popup does get blocked.

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https://github.com/w3c/permissions/issues/210