Define Browser interface

[oliverdunk]

Defines the Browser interface so that it can be referenced from other specifications which define WebExtension APIs.

Also, update heading levels for other content to correctly create table of contents.

[patrickkettner]

Shape LGTM, but there’s linting error that needs sorting

[domfarolino]

I spoke with @oliverdunk about this a little offline. It sounds like we want is interface objects annotated with [LegacyNoInterfaceObject] as this CL does, and probably [Replaceable]-annotated attributes on those objects, so that we can support what I understand extensions needs to support regarding attribute overwriting. But we should be absolutely sure we need this combination of things. For example:

• Do browser definitely not expose window.Browser as a non-constructible interface? If they expose this interface, then we don't need [LegacyNoInterface] objects. Same goes for other properties of [LegacyNoInterfaceObject]. Do existing browsers all match the prototype exposure quirks of LNIO, like https://webidl.spec.whatwg.org/#ref-for-LegacyNoInterfaceObject%E2%91%A0%E2%91%A1 and others?
• We should also verify that all browsers match the attribute assignment semantics that https://webidl.spec.whatwg.org/#Replaceable gives us. For example, the shadowing and deletion semantics in the example under this section: https://webidl.spec.whatwg.org/#Replaceable. Does that example exemplify the implementation that all browsers ship?

Finally, regarding the failing validation step, if we're hell-bent on preserving [LegacyNoInterfacObject] semantics in our spec, then we'll have to add VALIDATE_WEBIDL: false to https://github.com/w3c/webextensions/blob/main/.github/workflows/deploy.yml#L52. But it's strongly recommended that we try and align implementations to normal interface semantics, if we can. Over a period from 2018–2021, we removed all references to LNIO in all modern specs, which required some implementation changes as I understand it, so it'd be great if we could do the same for extensions. If not, we can live with it. We just to disable webidl validation.