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/test/fixtures/plugin-proposal-class-properties/loose-not-overwritten/input.js

@@ -0,0 +1,3 @@
+class A {
+  foo;
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/loose-not-overwritten/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": [
+    ["proposal-class-properties", { "loose": true }, "name 1"],
+    ["proposal-class-properties", { "loose": false }, "name 2"]
+  ]
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/loose-not-overwritten/output.js

@@ -0,0 +1,6 @@
+class A {
+  constructor() {
+    this.foo = void 0;
+  }
+
+}