Emit more modern code for browsers (#1614)

Adds a `browserlist` field to `package.json`, which Webpack picks up so
it doesn't convert nullish coalescing operators into obfuscated messes
like `key !== null && key !== void 0 ? key : null`.

This improves output size a little & improves the debugging experience
as well.

Tested in Chrome, FF, & Safari locally and didn't encounter any issues.

frontend/package.json

@@ -133,5 +133,8 @@
     "**/playwright": "1.32.1",
     "**/lit": "3.1.1",
     "@web/dev-server-esbuild/esbuild": "^0.19.5"
-  }
+  },
+  "browserslist": [
+    "defaults"
+  ]
 }