Open new slider constraint functions (#395)

Useful functions used to motorize the slider constraint.

As a reference, the bullet forklift demo use them here:
https://github.com/bulletphysics/bullet3/blob/master/examples/ForkLift/ForkLiftDemo.cpp#L803-L805

Thanks to these functions I expect to introduce motorized prismatic joints into BabylonJS: https://doc.babylonjs.com/divingDeeper/physics/joints#motors

README.md

@@ -163,8 +163,9 @@ This offers many advantages (keeping its native environment clean, portability,
 To do this, you just have to install Docker and run:
 
   ```bash
-  $ docker-compose build  # to create the Docker image
-  $ docker-compose up     # to create the Docker container and build ammo.js
+  $ docker-compose build        # to create the Docker image
+  $ docker-compose up           # to create the Docker container and build ammo.js
+  $ docker-compose run builder  # to build again the ammojs targets after any modification
   ```
 
 If you want to add arguments to cmake, you have to edit the `docker-compose.yml` file. 

ammo.idl

@@ -835,6 +835,10 @@ interface btSliderConstraint {
   void setUpperLinLimit(float upperLimit);
   void setLowerAngLimit(float lowerAngLimit);
   void setUpperAngLimit(float upperAngLimit);
+
+  void setPoweredLinMotor(boolean onOff);
+  void setMaxLinMotorForce(float maxLinMotorForce);
+  void setTargetLinMotorVelocity(float targetLinMotorVelocity);
 };
 btSliderConstraint implements btTypedConstraint;