Make @babel/plugin-class-features a normal helper package (#9083)

* Make @babel/plugin-class-features a normal helper package

This effectively disallows using it directly.

* Rename helper

* Style

* Don't add prefix to plugin name

* Move private methods plugin

packages/babel-helper-create-class-features-plugin/src/decorators.js

@@ -0,0 +1,3 @@
+export function hasDecorators(path) {
+  return !!(path.node.decorators && path.node.decorators.length);
+}