Fix babel warning about private-methods

The "loose" option must be the same for:

- @babel/plugin-proposal-class-properties,
- @babel/plugin-proposal-private-methods
- @babel/plugin-proposal-private-property-in-object

class-properties and private-methods are dependencies of
@babel/preset-env, which is required by webpacker.

This commit sets the same loose value for both class-properties and
private-methods to silence the warning 

Fix #3008

lib/install/config/babel.config.js

@@ -53,6 +53,12 @@ module.exports = function(api) {
           useBuiltIns: true
         }
       ],
+      [
+        '@babel/plugin-proposal-private-methods',
+        {
+          loose: true
+        }
+      ],
       [
         '@babel/plugin-transform-runtime',
         {

lib/install/examples/react/babel.config.js

@@ -62,6 +62,12 @@ module.exports = function(api) {
           useBuiltIns: true
         }
       ],
+      [
+        '@babel/plugin-proposal-private-methods',
+        {
+          loose: true
+        }
+      ],
       [
         '@babel/plugin-transform-runtime',
         {