Add support for enabling .mjs extension resolving (#6525)

### Description

Adds support for resolving `.mjs` files without the extension. This is
something we do currently in Next.js.

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### Testing Instructions

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crates/turbopack/src/resolve.rs

@@ -171,6 +171,9 @@ async fn base_resolve_options(
             ext.push(".jsx".to_string());
         }
         ext.push(".js".to_string());
+        if opt.enable_mjs_extension {
+            ext.push(".mjs".to_string());
+        }
         if opt.enable_node_native_modules {
             ext.push(".node".to_string());
         }

crates/turbopack/src/resolve_options_context.rs

@@ -25,6 +25,9 @@ pub struct ResolveOptionsContext {
     #[serde(default)]
     pub enable_node_native_modules: bool,
     #[serde(default)]
+    // Enable resolving of .mjs files without the .mjs extension
+    pub enable_mjs_extension: bool,
+    #[serde(default)]
     /// Enable resolving of the node_modules folder when within the provided
     /// directory
     pub enable_node_modules: Option<Vc<FileSystemPath>>,