feat: add Num.tan method to core library and documentation

Add the missing `tan` method to the Num class, implementing the tangent
trigonometric function. This fills the gap between the existing `sin` and
`cos` methods, providing a complete set of basic trigonometric operations.

- Register `num_tan` primitive in the VM core initialization
- Implement `DEF_PRIMITIVE(num_tan)` using C's `tan()` math function
- Add documentation entry for `tan` in the core library reference

doc/site/core/num.markdown

@@ -42,6 +42,10 @@ The sine of the number.
 
 The square root of the number. Returns `nan` if the number is negative.
 
+### **tan**
+
+The tangent of the number.
+
 ### **-** operator
 
 Negates the number.