Document how to extend turbo_stream for custom actions (hotwired#555)

Closes [hotwired#554][]

The current `Turbo::Streams::TagBuilder` class supports the actions that
Turbo provides out of the box.

For applications that define [Custom Stream Actions][], this new support
aims to provide them with a mechanism to define Rails-side helpers.

For example, consider an `[action="highlight"]` custom stream. To build
those with first-party tooling, applications could add a [hook block][]
to their initializer:

  ```ruby
  # config/initializers/turbo.rb
  ActiveSupport.on_load :turbo_streams_tag_builder do
    def highlight(target)
      action :highlight, target
    end

    def highlight_all(targets)
      action_all :highlight, targets
    end
  end

  turbo_stream.highlight "my-element"
  # => <turbo-stream action="highlight" target="my-element"><template></template></turbo-stream>

  turbo_stream.highlight_all ".my-selector"
  # => <turbo-stream action="highlight" targets=".my-selector"><template></template></turbo-stream>
  ```

[hotwired#554]: hotwired#554
[Custom Stream Actions]: https://turbo.hotwired.dev/handbook/streams#custom-actions
[hook block]: https://edgeapi.rubyonrails.org/classes/ActiveSupport/LazyLoadHooks.html#method-i-on_load

app/models/turbo/streams/tag_builder.rb

@@ -22,6 +22,24 @@
 #   <%= turbo_stream.append dom_id(topic_merge) do %>
 #     <%= link_to topic_merge.topic.name, topic_path(topic_merge.topic) %>
 #   <% end %>
+#
+# To integrate with custom actions, extend this class in response to the :turbo_streams_tag_builder load hook:
+#
+#   ActiveSupport.on_load :turbo_streams_tag_builder do
+#     def highlight(target)
+#       action :highlight, target
+#     end
+#
+#     def highlight_all(targets)
+#       action_all :highlight, targets
+#     end
+#   end
+#
+#   turbo_stream.highlight "my-element"
+#   # => <turbo-stream action="highlight" target="my-element"><template></template></turbo-stream>
+#
+#   turbo_stream.highlight_all ".my-selector"
+#   # => <turbo-stream action="highlight" targets=".my-selector"><template></template></turbo-stream>
 class Turbo::Streams::TagBuilder
   include Turbo::Streams::ActionHelper
 
@@ -246,4 +264,6 @@ def render_record(possible_record)
         @view_context.render(partial: record, formats: :html)
       end
     end
+
+  ActiveSupport.run_load_hooks :turbo_streams_tag_builder, self
 end

test/dummy/config/initializers/turbo.rb

@@ -0,0 +1,9 @@
+ActiveSupport.on_load :turbo_streams_tag_builder do
+  def highlight(target)
+    action :highlight, target
+  end
+
+  def highlight_all(targets)
+    action_all :highlight, targets
+  end
+end

test/streams/streams_helper_test.rb

@@ -3,6 +3,8 @@
 class TestChannel < ApplicationCable::Channel; end
 
 class Turbo::StreamsHelperTest < ActionView::TestCase
+  attr_accessor :formats
+
   test "with streamable" do
     assert_dom_equal \
       %(<turbo-cable-stream-source channel="Turbo::StreamsChannel" signed-stream-name="#{Turbo::StreamsChannel.signed_stream_name("messages")}"></turbo-cable-stream-source>),
@@ -33,4 +35,12 @@ class Turbo::StreamsHelperTest < ActionView::TestCase
       turbo_stream_from("messages", channel: "NonExistentChannel", data: {payload: 1})
   end
 
+  test "custom turbo_stream builder actions" do
+    assert_dom_equal <<~HTML.strip, turbo_stream.highlight("an-id")
+      <turbo-stream action="highlight" target="an-id"><template></template></turbo-stream>
+    HTML
+    assert_dom_equal <<~HTML.strip, turbo_stream.highlight_all(".a-selector")
+      <turbo-stream action="highlight" targets=".a-selector"><template></template></turbo-stream>
+    HTML
+  end
 end