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

wasm/codegen/generators/structs.py

@@ -381,6 +381,9 @@ def build_struct_header(
     builder.line(f"{s}* get() const;")
     builder.line(f"void set({s}* ptr);")
 
+    if w == "MjSpec":
+      builder.line("emscripten::val timer() const;")
+
     # field declarations
     for field in wrapped_fields:
       if field.declaration and field not in member_inits:
@@ -598,6 +601,7 @@ def _build_struct_bindings(
   #undef X_ACCESSOR""".lstrip())
     elif w == "MjSpec":
       builder.line(".constructor<const MjSpec &>()")
+      builder.line('.property("timer", &MjSpec::timer)')
     elif w == "MjvScene":
       builder.line(".constructor<MjModel *, int>()")
       builder.line(".constructor<>()")