Add Notifications API

[autonome]

• reviewed prior work, or confirmed not already in progress
• moved files
• updated filename if needed (eg remove "TC39-" or s/webdriver2/webdriver/)
• removed draft date
• determined name
• added description (triangulated from mdn, spec, caniuse)
• added entry point
• verified baselines
• checked api list against mdn list
• added to a group or not
• determined whether the feature should be split
• looked for other related draft features that could be included
• what else?

See also

#960 and #973

[autonome]

Wrote up the feature composition proposal here: #973 (comment)

We can consider a group once we get to push notifications.

[ddbeck]

I had a look at this and ended up poring over everything to do with notifications. It's very messy—primarily because caniuse describes this feature in a peculiar way—so we need to tread carefully as long as we're setting the caniuse key on this feature. See #973 (comment) for a much, much more detailed take.

[autonome]

Re-requested review from @captainbrosset for the description shortening in notifications-apps.yml.

[autonome]

This now has the BCD changes from mdn/browser-compat-data#24705 and implements the approach recommended in #973.

[ddbeck]

Let's go! Thank you @autonome and @captainbrosset for making this happen—a ton of work, but a huge improvement to the BCD and really exercising what we can do feature construction. 👏