Stream Tag Builder: Support :renderable arguments

[seanpdoyle]

When passed a valid :renderable option (like an object that responds to #render_in), treat that object as both the <turbo-stream> element's template contents and attempt to treat it as the target.

For example, consider a simplified "component" class:

class Component
  extend ActiveModel::Naming

  def initialize(id:, content:) = (@id, @content = id, content)
  def render_in(...) = @content
  def to_key = [@id]
end

component = Component.new(id: 1, content: "Hello, world")

turbo_stream.update(component) # => <turbo-stream action="update" target="component_1"><template>Hello, world</template></turbo-stream>

[seanpdoyle]

cc @swanson, since this started as a conversation on the Hotwired discord.