POC: Render a Slack Divider using a sigil macro

[jwright]

This is a proof of concept to be able to render the following Slack BlockKit using a custom sigil and renderer.

The output should look like this:

{
  "type": "home",
  "blocks": [
    {
      "type": "divider"
    }
  ]
}

The input should be defined using a new ChEEX sigil like this:

~C"""
<%= slack do %>
  <.divider />
<% end %>
"""

And it should be able to be rendered within a Slack view like this (??):

~C"""
<.slack.view blocks={[render_block(:divider)]} />
"""