Add public _ModifierStack type (#998)

* Make BuiltinRegistry and ModifierContainer public

* Revert "Make BuiltinRegistry and ModifierContainer public"

This reverts commit 4b68667.

* Add public `_ModifierStack` type

* Convert to ViewModifier

Author: carson-katri

Sources/LiveViewNative/ModifierStack.swift

@@ -0,0 +1,33 @@
+//
+//  ModifierStack.swift
+//
+//
+//  Created by Carson Katri on 6/29/23.
+//
+
+import SwiftUI
+
+/// A container that decodes a modifier stack.
+///
+/// Use this as a modifier:
+///
+/// ```swift
+/// content
+///     .modifier(modifierStack)
+/// ```
+public struct _ModifierStack<R: RootRegistry>: Decodable, ViewModifier {
+    private let stack: [ModifierContainer<R>]
+    
+    @ObservedElement private var element
+    @LiveContext<R> private var context
+    
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        self.stack = try container.decode([ModifierContainer<R>].self)
+    }
+    
+    public func body(content: Content) -> some View {
+        content
+            .applyModifiers(stack[...], element: element, context: context.storage)
+    }
+}

lib/live_view_native_swift_ui/types/modifier_stack.ex

@@ -0,0 +1,7 @@
+defmodule LiveViewNativeSwiftUi.Types.ModifierStack do
+  use LiveViewNativePlatform.Modifier.Type
+  def type, do: :map
+
+  def cast(%LiveViewNativeSwiftUi.Modifiers{} = value), do: {:ok, Jason.decode!(Jason.encode!(value))}
+  def cast(_), do: :error
+end