Introduce a `StreamsBuilder` class

[marcoroth]

Something like:

Turbo::StreamsBuilder.render do |turbo_stream|
  turbo_stream.append(...)
  turbo_stream.console_log(...)
  turbo_stream.morph(...)
end

Which would return one string like:

<turbo-stream action="append" ...>...</turbo-stream>
<turbo-stream action="console_log" ...>...</turbo-stream>
<turbo-stream action="morph" ...>...</turbo-stream>

or inside a model:

class Post < ApplicationRecord
  broadcast_turbo_stream_actions do |turbo_stream|
    turbo_stream.append(...)
    turbo_stream.console_log(...)
    turbo_stream.morph(...)
  end
end

[marcoroth]

A POC of this has been implemented marcoroth/turbo_power-rails#18, but in my option it makes more sense to have this class here in turbo-ruby.