Update internal version of terser to 4.8.0

I was hoping this version would make optional chaining work but sadly
it looks like we will need to update v5 for that to work.

See #15827

third_party/terser/README.md

@@ -1,4 +1,4 @@
-Bundled version of terser 4.1.0 with emscripten patches
+Bundled version of terser 4.8.0 with emscripten patches
 =======================================================
 
 We maintain a few downstream patches to terser which means we can't use the
@@ -13,5 +13,5 @@ To make changes to this code please submit patches to
 https://github.com/emscripten-core/terser/ and then re-create this bundle
 using the following steps:
 
-  $ npx rollup -c
-  $ cp dist/bundle.js $EMSCRIPTEN_ROOT/third_party/terser/terser.js
+  $ CI=1 npx rollup -c
+  $ cp dist/bundle.min.js $EMSCRIPTEN_ROOT/third_party/terser/terser.js