Update Class Fields to Stage 3 and change default behavior (#6076)

* add transform-class-properties to stage 3, set spec mode to default

* update readme with examples; use `buildUndefinedNode()`; change behavior to always define both static and nonstatic class properties regardless of spec/loose mode; update tests

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/class-prop-values-3/expected.js

@@ -2,6 +2,7 @@ class X {
   constructor() {
     this.a = 2;
     this.b = '3';
+    this.c = void 0;
   }
 
 }

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/class-prop-values-3/options.json

@@ -1,6 +1,6 @@
 {
   "plugins": [
     "transform-flow-comments",
-    "transform-class-properties"
+    ["transform-class-properties", {"loose": true}]
   ]
 }