Replace babel/polyfill w/ core-js v3.

Babel versions above 7.4 recommend using new core-js polyfill over
deprecated babel/polyfill. Also updated babel/core and babel/preset-env.
Added babelrc for defined config of corejs version & targeted browsers.

.babelrc

@@ -0,0 +1,15 @@
+{
+  presets: [
+   [
+     "@babel/preset-env",
+      {
+        modules: false,
+        targets: {
+          browsers: ["IE 11", "> 5%"]
+        },
+        useBuiltIns: "usage",
+        corejs: "3"
+      }
+    ]
+  ]
+}