feat: add List.sort and List.sortBy functions (issue 9)

Add sorting support to the List module across all backends:
- List.sort for natural ordering (Int, Float, String, Bool, Char)
- List.sortBy for custom comparator-based sorting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types.rs

@@ -1551,6 +1551,26 @@ impl TypeEnv {
                     Type::Unit,
                 ),
             ),
+            (
+                "sort".to_string(),
+                Type::function(
+                    vec![Type::List(Box::new(Type::var()))],
+                    Type::List(Box::new(Type::var())),
+                ),
+            ),
+            (
+                "sortBy".to_string(),
+                {
+                    let elem = Type::var();
+                    Type::function(
+                        vec![
+                            Type::List(Box::new(elem.clone())),
+                            Type::function(vec![elem.clone(), elem], Type::Int),
+                        ],
+                        Type::List(Box::new(Type::var())),
+                    )
+                },
+            ),
         ]);
         env.bind("List", TypeScheme::mono(list_module_type));