Only polyfill fetch for file: URLs on Node.

There are some cases where one might want to load a bundle from network
on Node, like in the VS Code extension host (at a future point, when
VS Code updates the Node version to v20.x or later and the extension
switches to using proper async imports instead of the awful hack it is
currently doing).

lib/fetch.js

@@ -1,13 +1,19 @@
-// Node v18.x doesn't have a usable `fetch()`.
-let fetch = globalThis.fetch;
+let fetch;
 if (typeof process === 'object' && process.release?.name === 'node') {
-    fetch = async function(url) {
-        const { readFile } = await import('fs/promises');
-        let contentType = 'application/octet-stream';
-        if (url.pathname.endsWith('.wasm'))
-            contentType = 'application/wasm';
-        return new Response(await readFile(url), { headers: { "Content-Type": contentType } });
+    // Node doesn't have a usable `fetch()`.
+    fetch = async function(url, options) {
+        if (url.protocol === 'file:') {
+            const { readFile } = await import('fs/promises');
+            let contentType = 'application/octet-stream';
+            if (url.pathname.endsWith('.wasm'))
+                contentType = 'application/wasm';
+            return new Response(await readFile(url), { headers: { "Content-Type": contentType } });
+        } else {
+            return globalThis.fetch(url, options);
+        }
     };
+} else {
+    fetch = globalThis.fetch;
 }
 
 export default fetch;