[mod] client/simple: client plugins (#5406)

* [mod] client/simple: client plugins

Defines a new interface for client side *"plugins"* that coexist with server
side plugin system. Each plugin (e.g., `InfiniteScroll`) extends the base
`ts Plugin`. Client side plugins are independent and lazy‑loaded via `router.ts`
when their `load()` conditions are met. On each navigation request, all
applicable plugins are instanced.

Since these are client side plugins, we can only invoke them once DOM is fully
loaded. E.g. `Calculator` will not render a new `answer` block until fully
loaded and executed.

For some plugins, we might want to handle its availability in `settings.yml`
and toggle in UI, like we do for server side plugins. In that case, we extend
`py Plugin` instancing only the information and then checking client side if
[`settings.plugins`](1ad832b1dc/client/simple/src/js/toolkit.ts (L134))
array has the plugin id.

* [mod] client/simple: rebuild static

client/simple/package.json

@@ -19,9 +19,7 @@
     "lint:tsc": "tsc --noEmit"
   },
   "browserslist": [
-    "Chrome >= 93",
-    "Firefox >= 92",
-    "Safari >= 15.4",
+    "baseline 2022",
     "not dead"
   ],
   "dependencies": {
@@ -38,6 +36,7 @@
     "edge.js": "~6.3.0",
     "less": "~4.4.2",
     "lightningcss": "~1.30.2",
+    "mathjs": "~15.0.0",
     "sharp": "~0.34.5",
     "sort-package-json": "~3.5.0",
     "stylelint": "~16.26.0",