Support GitLab multi-line blockquoting inside lists

What does this MR do and why?

This adds support for our multiline-blockquoting inside ordered and unordered lists, such as

- list item

  >>>
  quote
  >>>

How to set up and validate locally

Paste the following markdown into a comment box and preview it:

- list item

  >>>
  quoted text
  
  ```ruby
      def initialize(text, context = nil, result = nil)
        super text, context, result
        @text = @text.delete("\r")
      end
  ```
  >>>

- list item 2

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #331519 (closed)