Migrate pymdownx to pymdown_extensions

Resolves #68

This commit adds `pymdown_extensions` registration as copy of `pymdownx`
for use by plugins running on python 3.13 plugin_host.

Note: Those need to explicitly specify dependency using official pypi name:

```
{
	"*": {
		"*": [
			"pymdown_extensions"
		]
	}
}
```

Once sublimehq/package_control#1719 is merged, `pymdownx`
can be removed. It is required for compatibility until then.

Author: deathaxe

repository.json

@@ -1479,7 +1479,7 @@
 			]
 		},
 		{
-			"name": "pymdownx",
+			"name": "pymdown_extensions",
 			"description": "PyMdown Extensions for Python Markdown",
 			"author": "facelessuser",
 			"issues": "https://github.com/facelessuser/pymdown-extensions/issues",
@@ -1496,6 +1496,19 @@
 				}
 			]
 		},
+		{
+			"name": "pymdownx",
+			"description": "PyMdown Extensions for Python Markdown",
+			"author": "facelessuser",
+			"issues": "https://github.com/facelessuser/pymdown-extensions/issues",
+			"releases": [
+				{
+					"base": "https://github.com/facelessuser/sublime-pymdownx",
+					"python_versions": ["3.3", "3.8"],
+					"tags": "st3-"
+				}
+			]
+		},
 		{
 			"name": "pyserial",
 			"description": "Python serial port access library",