Upgrade to Babel 7 (rjsf-team#1322)

- Remove deprecated `babel-plugin-react-transform` with `@babel/plugin-transform-react-jsx`
- Update a few files with default exports as they were throwing warnings while running `dev`

.babelrc

@@ -1,20 +1,22 @@
 {
-  "presets": ["react", "env"],
+  "presets": [
+    "@babel/preset-react",
+    "@babel/preset-env"
+  ],
   "plugins": [
-    "transform-object-rest-spread",
-    "transform-class-properties",
-    "transform-runtime"
+    "@babel/plugin-proposal-object-rest-spread",
+    "@babel/plugin-proposal-class-properties",
+    [
+      "@babel/plugin-transform-runtime",
+      {
+        "corejs": 2
+      }
+    ]
   ],
   "env": {
     "development": {
       "plugins": [
-        ["react-transform", {
-          "transforms": [{
-            "transform": "react-transform-hmr",
-            "imports": ["react"],
-            "locals": ["module"]
-          }]
-        }]
+        "@babel/plugin-transform-react-jsx"
       ]
     }
   }