Markdown: Add full footnote spec to internal GLFM specs

What does this MR do and why?

Add additional tests from https://github.com/github/cmark-gfm/blob/master/test/extensions.txt into glfm_specification/input/gitlab_flavored_markdown/glfm_internal_extensions.md

How to set up and validate locally

Running scripts/glfm/verify-all-generated-files-are-up-to-date.rb should succeed (it's already run in the pipeline)

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 #390617 (closed)