Move ParameterView integration into context.parameter functions

[mjarvis]

Rather than providing parameter views in the environment:

• Allow passing a view: parameter to override default parameter views.
• Allow adding an extension to Context in order to add more default parameter views.

let foo: String = context.parameter(name: "Foo", defaultValue: "Bar") { name, value in
    Text("\(name): \(value)")
}

extension Context {
    func parameter(name: String, defaultValue: MyCustomType) -> MyCustomType {
        parameter(name: name, defaultValue: defaultValue) { name, value in
              // Default debug view for MyCustomType
        }
    }
}

Upsides:

• Easier attachment of custom parameter views
• Easier addition of custom types

Downsides:

• Cannot set new defaults over existing defaults.