feat: Implement built-in functions (len, type)

examples/built_ins.fem

@@ -0,0 +1,10 @@
+my_string is "Hello"
+my_list is [1, 2, 3]
+
+UwU Boy len(my_string)
+UwU Boy len(my_list)
+
+UwU Boy type(my_string)
+UwU Boy type(my_list)
+UwU Boy type(123)
+UwU Boy type(Kawaii)

src/interpreter.py

@@ -3,7 +3,9 @@ class Interpreter:
         self.ast = ast
         self.scope_stack = [{}]
         self.functions = {
-            "ask": self._ask_builtin
+            "ask": self._ask_builtin,
+            "len": self._len_builtin,
+            "type": self._type_builtin
         }
 
     def _ask_builtin(self, prompt):
@@ -174,6 +176,14 @@ class Interpreter:
     def visit_ReturnStatement(self, node):
         raise ReturnValue(self.visit(node.value))
 
+    @staticmethod
+    def _len_builtin(obj):
+        return len(obj)
+
+    @staticmethod
+    def _type_builtin(obj):
+        return str(type(obj).__name__)
+
 class ReturnValue(Exception):
     def __init__(self, value):
-        self.value = value
+        self.value = value