fix: add extends property in schema (lerna#4075)

Author: jhdm

packages/lerna/schemas/lerna-schema.json

@@ -9,6 +9,10 @@
       "type": "string",
       "description": "The JSON schema version used to validate this configuration file"
     },
+    "extends": {
+      "type": "string",
+      "description": "A shareable configuration preset that will be used as the base for this configuration"
+    },
     "version": {
       "type": "string",
       "description": "The version of the repository, or \"independent\" for a repository with independently versioned packages.",