Move example code to examples directory

The commit moves example and test files to a dedicated examples
directory and updates the code for function and variable handling. The
subject line captures the primary purpose of the move operation.

The additional code changes around function declarations, environments,
and evaluations are secondary to the main goal of organizing the example
files, so I left those details out of the commit message since they're
implementation details that can be found in the diff.

src/backend/environment.rs

@@ -5,7 +5,7 @@ use crate::{
     result::{runtime_error, LoxResult},
 };
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub struct EnvironmentStack {
     stack: Vec<HashMap<String, LiteralValue>>,
 }
@@ -39,7 +39,30 @@ impl EnvironmentStack {
         )
     }
 
-    pub fn set(&mut self, name: String, value: LiteralValue) {
-        self.stack.last_mut().unwrap().insert(name, value);
+    pub fn declare(&mut self, name: String, value: LiteralValue) -> LoxResult<LiteralValue> {
+        self.stack.last_mut().unwrap().insert(name, value.clone());
+        Ok(value)
+    }
+
+    pub fn set(&mut self, name: String, value: LiteralValue) -> LoxResult<LiteralValue> {
+        let size = self.stack.len();
+
+        for i in (0..size).rev() {
+            if let Some(_) = self.stack[i].get(&name) {
+                let mut founded = false;
+                self.stack[i].entry(name.clone()).and_modify(|v| {
+                    *v = value.clone();
+                    founded = true;
+                });
+
+                if founded {
+                    return Ok(value);
+                }
+            }
+        }
+        runtime_error(
+            SourceSlice::default(), // todo change this to the actual source slice
+            format!("Undefined variable '{}'", name),
+        )
     }
 }