[Drupal] Concatenate arrays that get rendered

[rbayliss]

Drupal supports rendering arrays of things. We don't, because the render system depends on too much other stuff. This PR implements some basic support for rendering something like:

{# @Component
name: Test
samples:
  render_array:
    content:
      field_image: 'Foo'
      field_body: 'Bar'
#}
<div>
  {{ content }}
</div>

Which would output <div>FooBar</div>.

To do:

• [ ] Docs review (can you figure out what is going on just from the documentation?)
• [ ] Concept review (does what we're doing even make sense)
• [ ] Code review (does the code work/is there a better way to do something)