Implement #use directive for dependency management

Introduces the `#use` preprocessor directive, allowing Myc scripts to
explicitly declare dependencies on other Myc libraries. This change
moves dependency management from a command-line argument to an in-script
declaration, ensuring scripts are self-contained and can correctly
resolve macros and other symbols.

Key features include:
- Declarations at the beginning of a file, evaluated before parsing.
- Support for relative paths and a new `->` separator.
- Automatic resolution of dependencies from specified search paths.
- Idempotent loading to prevent duplicate parsing and evaluation.
- No AST pollution; dependency management is a compile-time concern.

The compiler's lexer has been updated to recognize `#` as a comment
character, and the environment now manages search paths and loaded
modules. This lays the groundwork for more complex library structures
and improved code organization.

src/bin/ast.rs

@@ -45,12 +45,9 @@ fn main() {
     let mut env = Environment::new();
     env.optimization = !cli.no_opt;
     
-    // Load libraries
+    // Load libraries (Now just search paths)
     for lib_path in &cli.lib {
-        if let Err(e) = env.load_library(lib_path) {
-            eprintln!("❌ Error loading library {:?}: {}", lib_path, e);
-            std::process::exit(1);
-        }
+        env.add_search_path(lib_path);
     }
 
     if cli.bench || cli.update_bench {
@@ -77,6 +74,11 @@ fn main() {
     }
 
     if let Some(script_str) = cli.eval {
+        if let Err(e) = env.preload_dependencies(&script_str, None) {
+            eprintln!("❌ Error resolving dependencies: {}", e);
+            std::process::exit(1);
+        }
+
         if cli.dump {
             match env.dump_ast(&script_str) {
                 Ok(dump) => println!("{}", dump),
@@ -90,6 +92,11 @@ fn main() {
     } else if let Some(file_path) = cli.file {
         match fs::read_to_string(&file_path) {
             Ok(content) => {
+                if let Err(e) = env.preload_dependencies(&content, Some(&file_path)) {
+                    eprintln!("❌ Error resolving dependencies for {:?}:\n{}", file_path, e);
+                    std::process::exit(1);
+                }
+
                 if cli.dump {
                     match env.dump_ast(&content) {
                         Ok(dump) => println!("{}", dump),