feat($core): assert return type for functional plugin (#1516)

Author: shigma

packages/@vuepress/core/lib/node/plugin-api/util.js

@@ -43,6 +43,14 @@ exports.flattenPlugin = function (
     // 'Object.create' here is to give each plugin a separate context,
     // but also own the inheritance context.
     config = config(pluginOptions, Object.create(pluginContext), self)
+    const { valid, warnMsg } = assertTypes(config, [Object])
+    if (!valid) {
+      logger.warn(
+        `[${chalk.gray(shortcut)}] `
+        + `Invalid value for plugin: ${warnMsg}`
+      )
+      config = {}
+    }
   }
 
   // respect name in local plugin config