[Data Liberation] Add Markdown parsing libraries (#2092)

Adds a forked version of the markdown parsing libraries required by the
upcoming Markdown importer. We need out own fork for PHP 7.2
compatibility. The downgrade process was performed semi-automatically
via Rector.

This PR adds the following libraries:

* `league/commonmark`
* `webuni/front-matter`

There are no testing steps here. This PR only adds new code without
modifying the existing one.

A part of:

* #2080
* #1894

Author: adamziel

packages/playground/data-liberation-markdown/composer.json

@@ -0,0 +1,26 @@
+{
+	"name": "wordpress/data-liberation-markdown",
+	"prefer-stable": true,
+	"require": {
+		"league/commonmark": "^2.5",
+		"webuni/front-matter": "^2.0"
+	},
+	"config": {
+		"optimize-autoloader": true,
+		"preferred-install": "dist",
+		"allow-plugins": {
+			"dealerdirect/phpcodesniffer-composer-installer": true
+		}
+	},
+	"autoload": {
+		"classmap": [
+			"src/"
+		]
+	},
+	"authors": [
+		{
+			"name": "WordPress Contributors",
+			"email": "[email protected]"
+		}
+	]
+}