npm: Upgrade webpack related development dependencies

compression-webpack-plugin fixes https://www.npmjs.com/advisories/1548

Adjust to new copy-webpack-plugin API.

Thew new modules now create dist/index.html.gz in `NODE_ENV=production`
mode, so switch `WEBPACK_TEST` to index.css instead (which remains
uncompressed in both modes).

package.json

@@ -19,9 +19,9 @@
     "babel-eslint": "^10.0.3",
     "babel-loader": "^8.0.6",
     "chrome-remote-interface": "^0.28.0",
-    "compression-webpack-plugin": "^3.0.0",
-    "copy-webpack-plugin": "^5.0.3",
-    "css-loader": "^3.0.0",
+    "compression-webpack-plugin": "^6.0.0",
+    "copy-webpack-plugin": "^6.1.0",
+    "css-loader": "^4.3.0",
     "eslint": "^6.3.0",
     "eslint-config-standard": "^14.1.0",
     "eslint-config-standard-react": "^9.2.0",
@@ -35,13 +35,13 @@
     "eslint-plugin-standard": "^4.0.1",
     "htmlparser": "^1.7.7",
     "jed": "^1.1.1",
-    "mini-css-extract-plugin": "^0.9.0",
+    "mini-css-extract-plugin": "^0.11.0",
     "node-sass": "^4.14.1",
     "po2json": "^1.0.0-alpha",
     "qunit": "^2.9.3",
-    "sass-loader": "^8.0.2",
+    "sass-loader": "^10.0.2",
     "sizzle": "^2.3.3",
-    "stdio": "^0.2.7",
+    "stdio": "^2.1.0",
     "string-replace-loader": "^2.3.0",
     "webpack": "^4.35.3",
     "webpack-cli": "^3.3.5"