feat: Add support for if and compound statements.

src/ast.rs

@@ -67,5 +67,15 @@ pub enum Statement {
         value: Option<Expression>,
     },
 
+    If {
+        condition: Expression,
+        then: Box<Statement>,
+        elze: Option<Box<Statement>>,
+    },
+
+    Compound {
+        body: Vec<Statement>,
+    },
+
     Expr(Expression),
 }