Use webpack instead of rollup for the legacy build

This also changes the target path of the legacy build to build/legacy

config/webpack-config-legacy-build.js

@@ -0,0 +1,13 @@
+const path = require('path');
+module.exports = {
+  entry: './build/index.js',
+  devtool: 'source-map',
+  mode: 'production',
+  output: {
+    path: path.resolve('./build/legacy'),
+    filename: 'ol.js',
+    library: 'ol',
+    libraryTarget: 'umd',
+    libraryExport: 'default'
+  }
+};