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

introspect/enums.py

@@ -216,8 +216,9 @@ ENUMS: Mapping[str, EnumDecl] = dict([
          declname='enum mjtGain_',
          values=dict([
              ('mjGAIN_FIXED', 0),
-             ('mjGAIN_MUSCLE', 1),
-             ('mjGAIN_USER', 2),
+             ('mjGAIN_AFFINE', 1),
+             ('mjGAIN_MUSCLE', 2),
+             ('mjGAIN_USER', 3),
          ]),
      )),
     ('mjtBias',