Refactor math functions to a separate module

Move the `register_math` function and its associated logic into a new
`math` module. This improves organization and separation of concerns
within the RTL AST.

src/ast/rtl/mod.rs

@@ -1,6 +1,7 @@
 pub mod core;
 pub mod datetime;
 pub mod intrinsics;
+pub mod math;
 pub mod type_registry;
 
 use crate::ast::environment::Environment;
@@ -8,4 +9,5 @@ use crate::ast::environment::Environment;
 pub fn register(env: &Environment) {
     core::register(env);
     datetime::register(env);
+    math::register(env);
 }