Add support for native layouts

[supernintendo]

This PR adds layouts support for native platforms. To enable native layouts within a Phoenix application, add use LiveViewNative.Layouts to your app's layout module like so:

defmodule MyAppWeb.Layouts do
  use ElixirconfChatWeb, :html
  use LiveViewNative.Layouts

  embed_templates "layouts/*.html"
end

Now each time the embed_templates/1 macro is used, LiveView Native will look for a template in the same directory with the same name for each platform using its file extension. For example if your app includes :live_view_native_swift_ui as a Mix dependency, all of the following templates will be compiled:

• app.html.heex
• app.swiftui.heex
• root.html.heex
• root.swiftui.heex

LiveView Native will choose which layout to render automatically based on the type of client that connects:

layoutsdemo.mov

Some additional notes:

• Each template will be compiled using the configuration of the platform library it corresponds with (so for :swiftui your template will contain SwiftUI nodes like <HStack>, <VStack>, <Text> instead of HTML).
• Stylesheets can be included within layouts by using them on the layouts module, similar to how they're inherited in LiveViews and LiveComponents.
• A special element, csrf-token, is supported on all non-HTML platforms for setting the CRSF token (since native platforms might not support the meta tag. Include it in your root layout like so:

<csrf-token value={get_csrf_token()} />
<%= @inner_content %>

• Platform libraries may define default root and app layouts which will render when native-specific layouts are not provided by the end-user application. See liveview-native/live_view_native_platform@414b406 for more info.

Closes #44

[AZholtkevych]

@carson-katri could you please review this one?