fix: Remove unused polyfills and update build config

Removes polyfills for `Object.assign` and `Set` which, as far as I can tell, are unused in the
codebase. Updates `webpack` from 3 to 4. Replaces `babel-preset-es2015` with `babel-preset-env`.
Reduces the minified bundle size (`feather.min.js`) by **9.3kB** (72.4kB to 63.1kB). Does not change
any functionality of the library.

.babelrc

@@ -1,4 +1,13 @@
 {
-  "presets": ["es2015"],
-  "plugins": ["transform-object-rest-spread"]
+  "presets": [
+    [
+      "env",
+      {
+        "targets": {
+          "browsers": ["last 2 versions"]
+        }
+      }
+    ],
+    "stage-2"
+  ]
 }