Add source location tracking to AST nodes

The commit expands the source location tracking through the AST by:

1. Switching from raw Expr/Stmt nodes to AstNode wrappers containing
source slices 2. Updating interpreter and parser to preserve location
info through node traversal 3. Enhancing error reporting with richer
source context information

test_error.lox

@@ -0,0 +1,9 @@
+// Test script to verify error positioning
+var a = 5;
+var b = "hello";
+print 0;
+var result = a + b;  // This should cause a type error
+print 1;
+43 + "hello"; // This should cause a type error
+print result;
+print 46 + "world"; // This should cause a type error