Modify mj_PLUGIN_LIB_INIT to support multiple plugins in the same compilation unit.

Previously on MSVC we used DllMain to register plugins, you cannot have multiple definitions of DllMain in a single unit so you would get errors if you tried to register two plugins. This modifies the implementation to insert a function pointer into the C runtime initialization instead.

PiperOrigin-RevId: 896612678
Change-Id: I07732147b955d741c836acff6da986db0e9b9eff

plugin/sensor/register.cc

@@ -17,7 +17,7 @@
 
 namespace mujoco::plugin::sensor {
 
-mjPLUGIN_LIB_INIT {
+mjPLUGIN_LIB_INIT(sensor) {
   TouchGrid::RegisterPlugin();
 }