Add support for toolbar modifier and related types

[MaxDesiatov]

Relevant APIs:

public protocol ToolbarContent {
  associatedtype Body : ToolbarContent
  @ToolbarContentBuilder var body: Self.Body { get }
}

@_functionBuilder public struct ToolbarContentBuilder {
  public static func buildBlock<Content>(
    _ content: Content
  ) -> some ToolbarContent where Content : ToolbarContent
  
  public static func buildBlock<Content>(
    _ content: Content
  ) -> some CustomizableToolbarContent where Content : CustomizableToolbarContent
}

extension View {
  public func toolbar<Content>(
    @ViewBuilder content: () -> Content
  ) -> some View where Content : View
  
  public func toolbar<Content>(
    @ToolbarContentBuilder content: () -> Content
  ) -> some View where Content : ToolbarContent
  
  public func toolbar<Content>(
    id: Swift.String,
    @ToolbarContentBuilder content: () -> Content
  ) -> some View where Content : CustomizableToolbarContent
}

public struct ToolbarItem<ID, Content> : ToolbarContent where Content : View {
  public typealias Body = Never
}

extension ToolbarItem where ID == Void {
  public init(
    placement: ToolbarItemPlacement = .automatic,
    @ViewBuilder content: () -> Content
  )
}

extension ToolbarItem : CustomizableToolbarContent where ID == String {
  public init(
    id: String,
    placement: ToolbarItemPlacement = .automatic,
    showsByDefault: Swift.Bool = true,
    @ViewBuilder content: () -> Content
  )
}

extension ToolbarItem : Identifiable where ID : Hashable {
  public var id: ID { get }
}

public struct ToolbarItemGroup<Content> : ToolbarContent where Content : View {
  public init(
    placement: ToolbarItemPlacement = .automatic,
    @ViewBuilder content: () -> Content
  )
  public typealias Body = Never
}

public struct ToolbarItemPlacement {
  public static let automatic: Self
  public static let primaryAction: Self
  public static let confirmationAction: Self
  public static let cancellationAction: Self
  public static let destructiveAction: Self
  public static let navigationBarLeading: Self
  public static let navigationBarTrailing: Self
  public static let bottomBar: Self
}

extension View {
  public func toolbar<Content>(
    @ViewBuilder content: () -> Content
  ) -> some View where Content : View
  
  public func toolbar<Content>(
    @ToolbarContentBuilder content: () -> Content
  ) -> some View where Content : ToolbarContent
  
  public func toolbar<Content>(
    id: Swift.String,
    @ToolbarContentBuilder content: () -> Content
  ) -> some View where Content : CustomizableToolbarContent
}

[carson-katri]

I have a draft PR already open for this, just started rewriting it with the new preference key implementation.