Add support for plain text output (#1790)

lib/livebook/runtime.ex

@@ -45,8 +45,10 @@ defprotocol Livebook.Runtime do
           :ignored
           # IO output, adjacent such outputs are treated as a whole
           | {:stdout, binary()}
-          # Standalone text block
+          # Standalone text block otherwise matching :stdout
           | {:text, binary()}
+          # Plain text content
+          | {:plain_text, binary()}
           # Markdown content
           | {:markdown, binary()}
           # A raw image in the given format

lib/livebook_web/live/app_live.ex

@@ -251,7 +251,7 @@ defmodule LivebookWeb.AppLive do
   end
 
   defp filter_output({idx, output})
-       when elem(output, 0) in [:markdown, :image, :js, :control],
+       when elem(output, 0) in [:plain_text, :markdown, :image, :js, :control],
        do: {idx, output}
 
   defp filter_output({idx, {:tabs, outputs, metadata}}) do

lib/livebook_web/live/output.ex

@@ -47,6 +47,14 @@ defmodule LivebookWeb.Output do
     """
   end
 
+  defp render_output({:plain_text, text}, %{id: id}) do
+    assigns = %{id: id, text: text}
+
+    ~H"""
+    <div id={@id} class="text-gray-700"><%= @text %></div>
+    """
+  end
+
   defp render_output({:markdown, markdown}, %{id: id, session_id: session_id}) do
     live_component(Output.MarkdownComponent,
       id: id,