wip: Add an 'apply' button to relevant code snippets

[dustinbyrne]

This change:

• Adds an apply button to code blocks which follow a comment directive containing a file path, e.g.,

  <!-- file: app/models/user.rb -->
  ```ruby
  class User
  def self.find_by_id(id)
    # TODO: implement me
  end
  end

• Migrates code block frontend from inline strings to Vue components
• TODO: Highlight code in the code block instead of the user message?
• TODO: Improve the lifetime management of dynamic markdown Vue components (is this even necessary)?
• TODO: Test code block parsing and support nested code fences