Allow recursively nested tokens in attributes

[willcosgrove]

If you have a component that you want to take in an attribute of a token list, like class, and you want your interface to be polymorphic like an element method's attributes would be, you currently need to type check the provided attribute and handle it manually.

class Card < Phlex::HTML
  def initialize(class:)
    @class = grab(class:)
  end

  def view_template
    div(class: ["component-classes", @class])
  end
end

Before this PR, the above example will break if the person using the Card component tries to supply an array of classes, because Array is not a valid token type for __nested_tokens__.

render Card.new(class: "fine") # => <div class="component-classes fine"></div>
render Card.new(class: ["not-fine"]) # => 💥 

Now if an array is passed, it will also have __nested_tokens__ called on it.

render Card.new(class: ["now-fine"]) # => <div class="component-classes now-fine"></div>