Downgrade to size-plugin 2

Version 3 has some issues with Webpack 5; see
GoogleChromeLabs/size-plugin#40 . These rear
their head with the upcoming TypeScript config.

package.json

@@ -63,7 +63,7 @@
     "npm-run-all": "^4.1.5",
     "prettier": "^2.3.2",
     "process": "^0.11.10",
-    "size-plugin": "^3.0.0",
+    "size-plugin": "^2.0.2",
     "stream-browserify": "^3.0.0",
     "styled-jsx": "^3.4.4",
     "terser-webpack-plugin": "^5.1.1",