Remove usage of size_t types in mjModel, use mjtSize (uint64_t) instead to improve portabilty of MJB files to WASM.

PiperOrigin-RevId: 789321991
Change-Id: Ic25fab876c347fc75f5ca24cad09e8fd972f035f

src/engine/engine_util_misc.h

@@ -139,6 +139,9 @@ MJAPI int mju_isZero(mjtNum* vec, int n);
 // set integer vector to 0
 MJAPI void mju_zeroInt(int* res, int n);
 
+// set mjtSize vector to 0
+MJAPI void mju_zeroSize(mjtSize* res, size_t n);
+
 // set size_t vector to 0
 MJAPI void mju_zeroSizeT(size_t* res, size_t n);