LibJS: Support empty statements

We already skipped random semicolons in Parser::parse_program(), but now
they are properly matched and parsed as empty statements - and thus
recognized as a valid body of an if / else / while / ... statement.

Libraries/LibJS/AST.h

@@ -71,6 +71,12 @@ private:
 class Statement : public ASTNode {
 };
 
+class EmptyStatement final : public Statement {
+public:
+    Value execute(Interpreter&) const override { return js_undefined(); }
+    const char* class_name() const override { return "EmptyStatement"; }
+};
+
 class ErrorStatement final : public Statement {
 public:
     Value execute(Interpreter&) const override { return js_undefined(); }