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/main.rs

@@ -98,6 +98,10 @@ impl CompilerApp {
         self.env.optimization = true;
 
         let res = (|| -> Result<(myc::ast::types::Value, std::time::Duration), String> {
+            if let Err(e) = self.env.preload_dependencies(&self.source_code, self.current_example_path.as_deref()) {
+                return Err(format!("Dependency Error: {}", e));
+            }
+
             let start_run = std::time::Instant::now();
             let result = if self.debug_enabled {
                 let (res, logs) = self.env.run_debug(&self.source_code)?;
@@ -158,6 +162,12 @@ impl CompilerApp {
     fn dump_ast(&mut self) {
         self.env = Environment::new();
         self.env.optimization = true; // Enable optimization for AST dump
+        
+        if let Err(e) = self.env.preload_dependencies(&self.source_code, self.current_example_path.as_deref()) {
+            self.output_log = format!("Dependency Error: {}", e);
+            return;
+        }
+
         match self.env.dump_ast(&self.source_code) {
             Ok(dump) => {
                 self.output_log = format!("--- BOUND AST DUMP ---\n\n{}", dump);