Add vector.random_direction() (#14784)

Generate a random vector of unit length. Useful for many mods.

doc/lua_api.md

@@ -3820,6 +3820,8 @@ vectors are written like this: `(x, y, z)`:
       `vector.new(v)` does the same as `vector.copy(v)`
 * `vector.zero()`:
     * Returns a new vector `(0, 0, 0)`.
+* `vector.random_direction()`:
+    * Returns a new vector of length 1, pointing into a direction chosen uniformly at random.
 * `vector.copy(v)`:
     * Returns a copy of the vector `v`.
 * `vector.from_string(s[, init])`: