Add features for permissions policy and feature policy

[ddbeck]

This PR adds a feature for permissions policy and a discouraged feature for feature policy. This is a big mess, so these are draft features.

Unfortunately, BCD doesn't accurately record the data for these features and it it's a bit of work to unwind this. I propose that we add these as drafts for now and, at the time we merge this, file an issue on BCD that requests Feature-Policy and Permissions-Policy be separated into separate keys and see where things go from there.

The full backstory:

• The Feature-Policy header was once added to browsers.
• Later, the Feature-Policy was "renamed" to Permissions-Policy. "Renamed" because yes it was renamed, but it also got a breaking syntax change at the same time (see also Control browser features with Permissions Policy  |  Privacy & Security  |  Chrome for Developers).
• caniuse told this story by describing feature policy and permissions policy as separate features. In my opinion, this was the correct approach.
• BCD told this story by pretending that Feature-Policy was an alternative name for Permissions-Policy, ignoring the syntax change. I'm not quite sure why this happened, given the failure of such an approach earlier.

Given this story, we cannot accurately calculate either feature's implementation status in a way that accords with caniuse or, in my opinion, confusing developers (because let me tell you, I was confused about this and I knew most of the backstory already).

[ddbeck]

cc: @Elchi3 you might have opinions about this one, since I expect it to impact BCD as well.

[foolip]

I guess none of https://w3c.github.io/webappsec-permissions-policy/#idl-index has shipped...

[ddbeck]

Yeah. I asked about this at the time and it seems like at some point somebody thought about it but nothing else came of it.

[ddbeck]

I've sent mdn/browser-compat-data#26708 to start unwinding the compat data, to unblock this PR.