Add scope and basic stdlib for evaluation

Introduces a `Scope` struct for dynamic scope management and a `Context`
struct to hold the current scope.
Implements a basic standard library with arithmetic operations and a
greater-than comparison.
Modifies `Node::eval` to handle identifiers by resolving them within the
current scope and `Call` nodes for function execution.
Updates the parser to use `Context::new()` for evaluating vector
elements, ensuring a fresh scope.
Enhances the main loop to handle potential runtime errors during
evaluation.

src/main.rs

@@ -53,13 +53,19 @@ impl eframe::App for CompilerApp {
 
                     match parser.parse_expression() {
                         Ok(ast) => {
-                            let mut context = Context {};
-                            let result = ast.eval(&mut context);
-                            self.output_log = format!(
-                                "AST Parsed Successfully.\nResult: {}\n\nFinished at {:?}",
-                                result,
-                                std::time::SystemTime::now(),
-                            );
+                            let mut context = Context::new(); // Use new() which registers stdlib
+                            match ast.eval(&mut context) {
+                                Ok(result) => {
+                                    self.output_log = format!(
+                                        "AST Parsed & Evaluated Successfully.\nResult: {}\n\nFinished at {:?}",
+                                        result,
+                                        std::time::SystemTime::now(),
+                                    );
+                                }
+                                Err(e) => {
+                                    self.output_log = format!("Runtime Error: {}", e);
+                                }
+                            }
                         }
                         Err(e) => {
                             self.output_log = format!("Parser Error: {}", e);