Add Program node kind

The `Program` node kind is introduced to represent top-level code
blocks, distinguishing them from `Block` nodes which introduce new
scopes. This commit updates various compiler passes to handle the
`Program` node, ensuring correct AST traversal and processing.

The `Program` node is similar to `Block`, but its definitions propagate
to the enclosing scope, unlike `Block` which creates a new, isolated
scope. This distinction is important for how variables and functions are
resolved within the compiled code.

src/ast/compiler/analyzer.rs

@@ -53,7 +53,7 @@ impl<'a> Analyzer<'a> {
                 }
                 self.collect_globals(value);
             }
-            NodeKind::Block { exprs } => {
+            NodeKind::Block { exprs } | NodeKind::Program { exprs } => {
                 for e in exprs {
                     self.collect_globals(e);
                 }
@@ -241,6 +241,16 @@ impl<'a> Analyzer<'a> {
                 }
                 (NodeKind::Block { exprs: new_exprs }, p)
             }
+            NodeKind::Program { exprs } => {
+                let mut new_exprs = Vec::with_capacity(exprs.len());
+                let mut p = Purity::Pure;
+                for e in exprs {
+                    let em = self.visit(e.clone());
+                    p = p.min(em.ty.purity);
+                    new_exprs.push(Rc::new(em));
+                }
+                (NodeKind::Program { exprs: new_exprs }, p)
+            }
 
             NodeKind::Tuple { elements } => {
                 let mut new_elements = Vec::with_capacity(elements.len());
@@ -350,7 +360,7 @@ impl NodeExt for NodeKind<TypedPhase> {
                 f(callee);
                 f(args);
             }
-            NodeKind::Block { exprs } => {
+            NodeKind::Block { exprs } | NodeKind::Program { exprs } => {
                 for e in exprs {
                     f(e);
                 }