Upgrade webpack to version 5

The only noticeable changes are that the built files are now *slightly* smaller, and that Webpack now supports optional chaining and nullish coalescing without the need for Babel plugins.

gulpfile.js

@@ -217,18 +217,6 @@ function createWebpackConfig(defines, output) {
           options: {
             presets: skipBabel ? undefined : ["@babel/preset-env"],
             plugins: [
-              [
-                "@babel/plugin-proposal-nullish-coalescing-operator",
-                {
-                  loose: true,
-                },
-              ],
-              [
-                "@babel/plugin-proposal-optional-chaining",
-                {
-                  loose: true,
-                },
-              ],
               "@babel/plugin-transform-modules-commonjs",
               [
                 "@babel/plugin-transform-runtime",

package.json

@@ -3,8 +3,6 @@
   "version": "2.0.0",
   "devDependencies": {
     "@babel/core": "^7.11.6",
-    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.10.4",
-    "@babel/plugin-proposal-optional-chaining": "^7.11.0",
     "@babel/plugin-transform-modules-commonjs": "^7.10.4",
     "@babel/plugin-transform-runtime": "^7.11.5",
     "@babel/preset-env": "^7.11.5",
@@ -58,7 +56,7 @@
     "vinyl": "^2.2.1",
     "vinyl-fs": "^3.0.3",
     "web-streams-polyfill": "^3.0.0",
-    "webpack": "^4.44.2",
+    "webpack": "^5.0.0",
     "webpack-stream": "~6.1.0",
     "wintersmith": "^2.5.0",
     "yargs": "^11.1.1"