Add vector.dot and vector.cross

Mostly copied from MarkuBu's code

doc/lua_api.txt

@@ -2517,6 +2517,10 @@ For the following functions, `v`, `v1`, `v2` are vectors,
     * Returns in order minp, maxp vectors of the cuboid defined by `v1`, `v2`.
 * `vector.angle(v1, v2)`:
     * Returns the angle between `v1` and `v2` in radians.
+* `vector.dot(v1, v2)`
+    * Returns the dot product of `v1` and `v2`
+* `vector.cross(v1, v2)`
+    * Returns the cross product of `v1` and `v2`
 
 For the following functions `x` can be either a vector or a number: