fix: Fix tree shaking even more (vercel/turborepo#8864)

### Description

Fix server action and rename the injected variable.

The previous code fails if the name of the top-level variable is `v`.
This was not a problem for almost all modules, but some packages like
[`@firebase/webchannel-wrapper`](https://unpkg.com/@firebase/webchannel-wrapper@0.2.41/dist/index.esm.js)
is published after mangling, so it can be a problem



### Testing Instructions

See #66689

crates/turbopack-ecmascript/src/references/esm/export.rs

@@ -442,8 +442,9 @@ impl CodeGenerateable for EsmExports {
                 EsmExport::LocalBinding(name, mutable) => {
                     if *mutable {
                         Some(quote!(
-                            "([() => $local, (v) => $local = v])" as Expr,
-                            local = Ident::new((name as &str).into(), DUMMY_SP)
+                            "([() => $local, ($new) => $local = $new])" as Expr,
+                            local = Ident::new((name as &str).into(), DUMMY_SP),
+                            new = Ident::new(format!("{name}_new_value").into(), DUMMY_SP),
                         ))
                     } else {
                         Some(quote!(
@@ -470,8 +471,9 @@ impl CodeGenerateable for EsmExports {
                         });
                         if *mutable {
                             quote!(
-                                "([() => $expr, (v) => $expr = v])" as Expr,
+                                "([() => $expr, ($new) => $expr = $new])" as Expr,
                                 expr: Expr = expr,
+                                new = Ident::new(format!("{name}_new_value").into(), DUMMY_SP),
                             )
                         } else {
                             quote!(

crates/turbopack-ecmascript/src/tree_shake/asset.rs

@@ -175,7 +175,7 @@ impl Module for EcmascriptModulePartAsset {
 impl Asset for EcmascriptModulePartAsset {
     #[turbo_tasks::function]
     fn content(&self) -> Vc<AssetContent> {
-        todo!()
+        self.full_module.content()
     }
 }