Add schema for contributed markdown API

Fixes #31874

extensions/markdown/package.json

@@ -234,7 +234,13 @@
 				"editor.wordWrap": "on",
 				"editor.quickSuggestions": false
 			}
-		}
+		},
+		"jsonValidation": [
+			{
+				"fileMatch": "package.json",
+				"url": "./schemas/package.schema.json"
+			  }
+		]
 	},
 	"scripts": {
 		"vscode:prepublish": "node ../../node_modules/gulp/bin/gulp.js --gulpfile ../../build/gulpfile.extensions.js compile-extension:markdown ./tsconfig.json",

extensions/markdown/schemas/package.schema.json

@@ -0,0 +1,32 @@
+{
+	"$schema": "http://json-schema.org/draft-04/schema#",
+	"title": "Markdown contributions to package.json",
+	"type": "object",
+	"properties": {
+		"contributes": {
+			"type": "object",
+			"properties": {
+				"markdown.previewStyles": {
+					"type": "array",
+					"description": "Contributed CSS files that change the look or layout of the Markdown preview",
+					"items": {
+						"type": "string",
+						"description": "Extension relative path to a css file"
+					}
+				},
+				"markdown.previewScripts": {
+					"type": "array",
+					"description": "Contributed scripts that are executed in the Markdown preview",
+					"items": {
+						"type": "string",
+						"description": "Extension relative path to a JavaScript file"
+					}
+				},
+				"markdown.markdownItPlugins": {
+					"type": "boolean",
+					"description": "Does this extension contributes a markdown-it plugin?"
+				}
+			}
+		}
+	}
+}