fix(babel-plugin-extend-docs): use @babel/core instead of babel-core

packages/babel-plugin-extend-docs/package.json

@@ -30,5 +30,8 @@
     "src",
     "test-node",
     "*.js"
-  ]
+  ],
+  "devDependencies": {
+    "@babel/core": "^7.10.1"
+  }
 }

packages/babel-plugin-extend-docs/test-node/helpers.js

@@ -1,4 +1,5 @@
-const babel = require('babel-core');
+// eslint-disable-next-line import/no-unresolved
+const babel = require('@babel/core');
 const babelPluginExtendDocs = require('../src/babelPluginExtendDocs.js');
 
 function executeBabel(input, options) {