Simplify Ignite: Eliminate PageContext by incorporating it into EnvironmentValues

[JPToroDev]

PageContext is essentially the same exact object as EnvironmentStore—a singleton that creates a context for the current page being published, and infuses that context with relevant values.

This similarity gives us the opportunity to consolidate these objects and make creating layouts even simpler.

This PR is the first in a larger series, and while it contains several changes, they are all in service of simplifying this code:

struct MainLayout: Layout {
   var body: some HTML {
       HTMLDocument {
           HTMLHead(for: page)
           HTMLBody {
               NavBar()
               Section(page.body)
               Section {
                   SocialFooter()
                   IgniteFooter()
               }
           }
           .padding(.vertical, 80)
       }
   }
}

Into this code:

struct MainLayout: Layout {
   var body: some HTML {
       Body {
           NavBar()
           Section(page.body)
           Section {
               SocialFooter()
               IgniteFooter()
           }
       }
       .padding(.vertical, 80)
   }
}

Head and Body now mirror scenes in SwiftUI, and the default Head is automatically provided by the result builder, hence its omission above. Head needs to be included only if you want extra head elements like Analytics.

The superficial changes:

• Restored HTMLHead and HTMLBody to Head and Body by reworking the associated type names that originally confused the compiler. Renamed HTMLDocument to Root.
• Added an internal environment default property to RootHTML

The main change is adding a page property to EnvironmentValues, which we access in the initializers of Head and Body now, instead of requiring a page as an input.

The final PR of this series will thoroughly polish variable and type names. The important thing here is functionality.

[twostraws]

I'm not sure about the name Root. Document, perhaps? Or something else?

[JPToroDev]

What's the hesitation with Root? That'd be the most accurate HTML terminology for it and complement RootElement nicely.