Add 'ParsoidModules' as a top-level core attribute in extension.json

See also I20f641a1ff032a6da3549b01dfaf8f4cf1eb5071, which uses this
attribute, and the earlier discussion in the (abandoned)
Ia251d631febf5be7fcad21cd8b73a68ec2785137.

Bug: T133320
Change-Id: I6c74938883376ec17f3790678b435585083a440f

docs/extension.schema.v2.json

@@ -1021,6 +1021,76 @@
 				}
 			}
 		},
+		"ParsoidModules": {
+			"type": "array",
+			"description": "List of extension modules to be registered with the Parsoid wikitext parser",
+			"items": {
+				"oneOf": [
+					{
+						"type": "string",
+						"description": "The fully-qualified name of a class implementing Wikimedia\\Parsoid\\Ext\\ExtensionModule"
+					},
+					{
+						"type": "object",
+						"description": "An object factory specification for a class implementing Wikimedia\\Parsoid\\Ext\\ExtensionModule",
+						"properties": {
+							"factory": {
+								"type": ["string", "array"],
+								"description": "A factory function to be called to create an ExtensionModule. This should be omitted if a class is specified."
+							},
+							"class": {
+								"type": "string",
+								"description": "The fully-qualified class name of a class implemeting Wikimedia\\Parsoid\\Ext\\ExtensionModule. This should be omitted if a factory is specified."
+							},
+							"args": {
+								"type": "array",
+								"description": "The arguments passed to the ExtensionModule constructor or factory"
+							}
+						}
+					},
+					{
+						"type": "object",
+						"description": "A Parsoid extension module configuration array",
+						"properties": {
+							"domProcessors": {
+								"type": "array",
+								"items": {
+									"description": "Object factory specification of a class implementing Wikimedia\\Parsoid\\Ext\\DOMProcessor"
+								}
+							},
+							"styles": {
+								"type": "array",
+								"items": {
+									"type": "string",
+									"description": "Additional ResourceLoader styles to include"
+								}
+							},
+							"tags": {
+								"type": "array",
+								"items": {
+									"type": "object",
+									"description": "An extension tag",
+									"properties": {
+										"name": {
+											"type": "string",
+											"description": "The name of the extension tag",
+											"required": "true"
+										},
+										"handler": {
+											"description": "An object factory specification of a class extending Wikimedia\\Parsoid\\Ext\\ExtensionTagHandler"
+										},
+										"options": {
+											"type": "object",
+											"description": "Additional Parsoid processing configuration for this extension tag"
+										}
+									}
+								}
+							}
+						}
+					}
+				]
+			}
+		},
 		"attributes": {
 			"description":"Registration information for other extensions",
 			"type": "object",

includes/registration/ExtensionProcessor.php

@@ -63,6 +63,7 @@ class ExtensionProcessor implements Processor {
 	 * @var string[]
 	 */
 	protected const CORE_ATTRIBS = [
+		'ParsoidModules',
 		'RestRoutes',
 		'SkinOOUIThemes',
 		'SearchMappings',