Add visualisation flag for activations.

- Added illustrative model: `test/engine/testdata/activation.xml`

PiperOrigin-RevId: 453628961
Change-Id: I061ec3d17e57c6baffa0fe446a0be3564c44519d

src/engine/engine_vis_init.c

@@ -68,8 +68,9 @@ const char* mjVISSTRING[mjNVISFLAG][3] = {
   {"Convex &Hull",    "0", "H"},
   {"Te&xture",        "1", "X"},
   {"&Joint",          "0", "J"},
-  {"Act&uator",       "0", "U"},
   {"Camera",          "0", "Q"},
+  {"Act&uator",       "0", "U"},
+  {"Act&ivation",     "0", ","},
   {"Light",           "0", "Z"},
   {"Tendon",          "1", "V"},
   {"Range Finder",    "1", "Y"},