Add experimental.strictNextHead to Turbopack (#55915)

This option only has a process.env replacement. Ensures
`test/development/client-navigation` can run for Turbopack.

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Author: timneutkens

packages/next-swc/crates/next-core/src/env.rs

@@ -61,6 +61,15 @@ pub async fn env_for_js(
         map.insert("__NEXT_STRICT_MODE_APP".to_string(), "true".to_string());
     }
 
+    map.insert(
+        "__NEXT_STRICT_NEXT_HEAD".to_string(),
+        if next_config.experimental.strict_next_head.unwrap_or(false) {
+            "true".to_string()
+        } else {
+            "false".to_string()
+        },
+    );
+
     if !test_mode.is_empty() {
         map.insert("__NEXT_TEST_MODE".to_string(), "true".to_string());
     }

packages/next-swc/crates/next-core/src/next_config.rs

@@ -403,6 +403,7 @@ pub enum LoaderItem {
 #[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize, TraceRawVcs)]
 #[serde(rename_all = "camelCase")]
 pub struct ExperimentalConfig {
+    pub strict_next_head: Option<bool>,
     pub server_components_external_packages: Option<Vec<String>>,
     pub turbo: Option<ExperimentalTurboConfig>,
     pub allowed_revalidate_header_keys: Option<Vec<String>>,

packages/next/src/lib/turbopack-warning.ts

@@ -31,6 +31,7 @@ const supportedTurbopackNextConfigOptions = [
   'assetPrefix',
   'distDir',
   'experimental.serverComponentsExternalPackages',
+  'experimental.strictNextHead',
   'experimental.turbo',
   'experimental.mdxRs',
   'experimental.forceSwcTransforms',