Add <damper> actuator shortcut and related mjGAIN_AFFINE.

**Rationale**

A general `mjGAIN_AFFINE` actuator adds the term `force = (c + kp * length + kv * velocity) * ctrl`. The damper shortcut restricts it to `kv * velocity * ctrl` and sets the `ctrllimited` attribute to true.

**API**

```xml
<actuator>
  <damper ctrlrange="0 1" kv="1"/>
</actuator>
```

* Required attributes: `ctrlrange` (>=0)
* Optional attributes: `kv` (>=0)

PiperOrigin-RevId: 455366866
Change-Id: I88773abbeccabb6442f215c9fc05e205df63f692

src/engine/engine_forward.c

@@ -197,6 +197,10 @@ void mj_fwdActuation(const mjModel* m, mjData* d) {
       gain = prm[0];
       break;
 
+    case mjGAIN_AFFINE:             // affine: prm = [const, kp, kv]
+      gain = prm[0] + prm[1]*d->actuator_length[i] + prm[2]*d->actuator_velocity[i];
+      break;
+
     case mjGAIN_MUSCLE:             // muscle gain
       gain = mju_muscleGain(d->actuator_length[i],
                             d->actuator_velocity[i],