Refactor thread pool implementation.

- Make mjTask non-opaque and remove C++ Task class.
- Make C++ thread pool a subclass of a skeletal mjThreadPool C struct.
- Make the mjTask status enum more consistent with the rest of MuJoCo.
- Change mju_threadPoolEnqueue to take just the mjTask. Users must now prepare the mjTask by assigning the function pointer and argument into the struct.
- Rename files in thread/ to be more consistent with the rest of MuJoCo.
- Run threading tests in CMake.
- Allow use of C++20 designated initializers.

Otherwise the functionality remains identical.

PiperOrigin-RevId: 564374675
Change-Id: I37c9894566bc39faf217e5aa97a4e2713a70e467

introspect/codegen/generate_enums.py

@@ -56,7 +56,10 @@ class MjEnumVisitor:
       child_kind = child.get('kind')
       if child_kind == 'EnumConstantDecl':
         next_idx = values[-1][1] + 1 if values else 0
-        value = int(child['inner'][0].get('value', next_idx))
+        if 'inner' in child:
+          value = int(child['inner'][0].get('value', next_idx))
+        else:
+          value = next_idx
         values.append((child['name'], value))
     return ast_nodes.EnumDecl(name=name, declname=name, values=dict(values))