Add language specification document

This commit introduces the BNF for the Myc language, along with its core
semantics, data types, and evaluation logic. It also details the macro
system and provides an overview of the standard library. This document
serves as a foundational context for LLMs to understand and generate Myc
code.

src/ast/rtl/core.rs

@@ -7,6 +7,7 @@ pub fn register(env: &Environment) {
     register_arithmetic(env);
     register_comparison(env);
     register_logic(env);
+    register_io(env);
 }
 
 fn register_constants(env: &Environment) {
@@ -489,3 +490,17 @@ fn register_logic(env: &Environment) {
         }
     });
 }
+
+fn register_io(env: &Environment) {
+    let print_ty = StaticType::Function(Box::new(Signature {
+        params: StaticType::Any,
+        ret: StaticType::Void,
+    }));
+    env.register_native_fn("print", print_ty, Purity::Impure, |args| {
+        for arg in args {
+            print!("{}", arg);
+        }
+        println!();
+        Value::Void
+    });
+}