feat(rust): add OutputFormat::Cjs (#358)

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### Description

Part of #45.

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### Test Plan

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---

crates/rolldown/src/bundler/chunk/render_chunk_exports.rs

@@ -27,6 +27,9 @@ impl Chunk {
               });
             return Some(MagicString::new(format!("export default {wrap_ref_name}();\n")));
           }
+          OutputFormat::Cjs => {
+            unreachable!("entry CJS should not be wrapped in `OutputFormat::Cjs`")
+          }
         }
       }
     }
@@ -94,6 +97,9 @@ impl Chunk {
           OutputFormat::Esm => {
             return vec!["default".to_string()];
           }
+          OutputFormat::Cjs => {
+            unreachable!("entry CJS should not be wrapped in `OutputFormat::Cjs`")
+          }
         }
       }
     }

crates/rolldown/src/bundler/options/output_options.rs

@@ -5,6 +5,7 @@ use super::file_name_template::FileNameTemplate;
 #[derive(Debug)]
 pub enum OutputFormat {
   Esm,
+  Cjs,
 }
 
 #[derive(Derivative)]