Markdown block: add support for footnotes.

[jeherve]

Fixes #11849

Proposed changes:

Jetpack ships with 2 flavors of Markdown:

• The Markdown feature available in the classic editor or in a classic block, and made available via our Markdown module:

jetpack/projects/plugins/jetpack/modules/markdown.php

Lines 3 to 4 in 3a3c4ad

	* Module Name: Markdown
	* Module Description: Write posts or pages in plain-text Markdown syntax

• The Markdown feature available to all Jetpack users in the block editor, via the Markdown block.

This PR is about the latter. It makes no changes to the classic Markdown feature available in the classic editor. This classic Markdown feature already supports footnotes.

The block, however, did not support footnotes, as it relies on a package (markdown-it) that does not support footnotes out of the box. There, you must use plugins to enable extra features.

In this PR, I'm bringing in a new dependency to the Jetpack plugin, to add support for footnotes:

• https://github.com/markdown-it/markdown-it-footnote
• MIT License

Note
Once this is merged, we will need to make changes to our support documentation to indicate that the Markdown block now supports footnotes.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

See #18035 and #11881.

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Start with a site that's connected to WordPress.com or in Offline mode.
• Go to Posts > Add New
• Add a Markdown block
• Add some content to that block. Possibly something quite long.
• In there, add 2 types of footnotes as highlighted in the docs here.
  • If you'd like, you can use the test content I've added below.
• Ensure those work well when saving your draft and viewing your changes on the frontend.

Example content

Once upon a time, there was a man named Jack who loved nothing more than cooking up a delicious meal for his loved ones. One day, he decided to prepare [his famous burgers and fries](https://shakeshack.com/) for his family. He spent hours in the kitchen, carefully selecting the freshest ingredients and crafting each burger with love and care.[^1]

As the scent of sizzling beef filled the air, Jack's family gathered around the kitchen, eagerly awaiting the meal. Finally, the burgers were ready and Jack proudly presented them to his loved ones. They took a bite and immediately their faces lit up with delight. "**This is amazing!**" they exclaimed.

But as they continued to eat, something strange began to happen. Jack's family members started to sprout feathers and wings.[^longnote] Before he could react, they had transformed into a flock of birds and flew out of the open window. Jack stood there, stunned, watching as his loved ones disappeared into the distance.

For days, Jack searched for his family, but they were nowhere to be found. Finally, he gave up and resigned himself to the fact that his cooking had somehow turned his family into birds. But despite his grief, he couldn't help but feel a sense of pride. After all, his cooking had been so delicious that it had caused his loved ones to take flight - *quite literally*.

From that day forward, Jack became known as the "bird chef," renowned throughout the land for his incredible burgers and fries that had the power to transform even the most ordinary person into a magnificent bird. And although he never did find his lost family, he took solace in knowing that they were out there somewhere, soaring through the skies, thanks to the power of his cooking.

[^1]: Check [the recipe here](https://www.myrecipes.com/recipe/classic-burger).

[^longnote]: This gets weird from here.

    I've been using ChatGPT at times to help me come up with random content, and this is what happens. I think this quote from ["Cheating is All You Need"](https://about.sourcegraph.com/blog/cheating-is-all-you-need) is appropriate here:

    > A raw LLM is like a Harvard CS grad who knows a lot about coding and took a magic mushroom about 4 hours ago, so it’s mostly worn off, but not totally.

[github-actions]

Thank you for your PR!

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Jetpack plugin:

• Next scheduled release: May 2, 2023.
• Scheduled code freeze: April 24, 2023.

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run

bin/jetpack-downloader test jetpack update/markdown-block-footnotes

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/markdown-block-footnotes

to get started. More details: p9dueE-5Nn-p2

[fgiannar]

I tested using the provided content on a JN site and my WPCOM Simple site.

Before (Simple site):

After (Simple site):

Before (JN site):

After (JN site):