add quat Lua library

doc/en/scripting/builtins/libquat.md

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+# *quat* library
+
+Quaternions manipulation library.
+
+## Quaternion from matrix - *mat4.from_quat(...)*
+
+```lua
+-- creates a quaternion based on the rotation matrix
+quat.from_mat4(m: matrix)
+
+-- writes a quaternion from the rotation matrix to dst
+quat.from_mat4(m: matrix, dst: quat)
+```
+
+## Spherical linear interpolation - *quat.slerp(...)*
+
+The interpolation always take the short path and the rotation is performed at constant speed.
+
+```lua
+-- creates a quaternion as an interpolation between a and b,
+-- where t is interpolation factor
+quat.slerp(a: quat, b: quat, t: number)
+
+-- writes a quaternion as an interpolation between a and b to dst,
+-- where t is interpolation factor
+quat.slerp(a: quat, b: quat, t: number, dst: quat)
+```
+
+## Casting to string - *quat.tostring(...)*
+
+```lua
+-- returns a string representing the contents of the quaternion
+quat.tostring(q: quat)
+```