Add compiler timing diagnostics to mjsCompiler, printed by compile.cc

For example, `compile mujoco_menagerie/robotis_op3/scene.xml` now outputs

```
Compile 1 (cold cache):
  total:      317.2 ms
  assets:     284.8 ms (wall clock)
    load:     616.1 ms
    hull:      26.4 ms
    poly:     137.8 ms
    inert:    177.8 ms
    bvh:      568.2 ms
    octr:       1.6 ms
    tex:       25.3 ms
  other:       32.4 ms

Compile 2 (warm cache):
  total:       79.9 ms
  assets:      54.5 ms (wall clock)
    load:     888.5 ms
    hull:       0.0 ms
    poly:       0.0 ms
    inert:      0.0 ms
    bvh:        0.0 ms
    octr:       0.0 ms
    tex:       21.4 ms
  other:       25.3 ms
```

PiperOrigin-RevId: 917850214
Change-Id: Iaec86230bec0faf2e47820e20cbff61de5b2621e

src/user/user_api.cc

@@ -427,6 +427,17 @@ const char* mjs_getError(mjSpec* s) {
 
 
 
+// get compiler timers from model
+const double* mjs_getTimer(mjSpec* s) {
+  if (!s) {
+    return nullptr;
+  }
+  mjCModel* modelC = static_cast<mjCModel*>(s->element);
+  return modelC->timer;
+}
+
+
+
 // check if model has warnings
 int mjs_isWarning(mjSpec* s) {
   mjCModel* modelC = static_cast<mjCModel*>(s->element);