Markdown block: add support for footnotes.

Fixes #11849

Also see #18035 and #11881.

This involves bringing in a new dependency to the Jetpack plugin:

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

projects/plugins/jetpack/changelog/update-markdown-block-footnotes

@@ -0,0 +1,4 @@
+Significance: patch
+Type: enhancement
+
+Markdown Block: add support for footnotes

projects/plugins/jetpack/extensions/blocks/markdown/renderer.js

@@ -2,11 +2,12 @@ import { __experimentalGetSpacingClassesAndStyles as getSpacingClassesAndStyles
 import { RawHTML } from '@wordpress/element';
 import { __ } from '@wordpress/i18n';
 import MarkdownIt from 'markdown-it';
+import footnote_plugin from 'markdown-it-footnote';
 
 /**
  * Module variables
  */
-const markdownConverter = new MarkdownIt();
+const markdownConverter = new MarkdownIt().use( footnote_plugin );
 const handleLinkClick = event => {
 	if ( event.target.nodeName === 'A' ) {
 		const hasConfirmed = window.confirm(

projects/plugins/jetpack/package.json

@@ -90,6 +90,7 @@
 		"jsdom": "20.0.3",
 		"mapbox-gl": "1.13.0",
 		"markdown-it": "12.3.2",
+		"markdown-it-footnote": "3.0.3",
 		"photon": "4.0.0",
 		"postcss-custom-properties": "12.1.7",
 		"prop-types": "15.7.2",