chore: add standalone function deprecation helper (#16782)

lib/common/api/deprecate.js

@@ -35,6 +35,14 @@ const deprecate = {
     }
   },
 
+  function: (fn, newName) => {
+    const warn = warnOnce(fn.name, newName)
+    return function () {
+      warn()
+      fn.apply(this, arguments)
+    }
+  },
+
   event: (emitter, oldName, newName) => {
     const warn = newName.startsWith('-') /* internal event */
       ? warnOnce(`${oldName} event`)