feat: Add support for expression and let statements.

src/ast.rs

@@ -58,3 +58,14 @@ pub enum BinaryOp {
     Assign,
     Member,
 }
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Statement {
+    Let {
+        name: Box<str>,
+        name_span: Span,
+        value: Option<Expression>,
+    },
+
+    Expr(Expression),
+}