feat: Add support for return and break statements.

src/ast.rs

@@ -68,15 +68,26 @@ pub enum Statement {
     },
 
     If {
+        kw_span: Span,
         condition: Expression,
         then: Box<Statement>,
         elze: Option<Box<Statement>>,
     },
 
     Loop {
+        kw_span: Span,
         body: Box<Statement>,
     },
 
+    Break {
+        kw_span: Span,
+    },
+
+    Return {
+        kw_span: Span,
+        value: Option<Expression>,
+    },
+
     Compound {
         body: Vec<Statement>,
     },