Add cross-runtime which wrapper utility

Add internal/which.ts that uses Bun.which when running on Bun,
and falls back to spawning the `which` command on other runtimes
like Node.js. This enables projects to work in both environments
without spawning unnecessary processes when Bun's native which
is available.

src/js/internal-for-testing.ts

@@ -165,6 +165,7 @@ export const bindgen = $zig("bindgen_test.zig", "getBindgenTestFunctions") as {
 
 export const noOpForTesting = $cpp("NoOpForTesting.cpp", "createNoOpForTesting");
 export const Dequeue = require("internal/fifo");
+export const which = require("internal/which").which;
 
 export const fs = require("node:fs/promises").$data;
 

src/js/internal/which.ts

@@ -0,0 +1,27 @@
+/**
+ * Cross-runtime `which` utility.
+ * Uses Bun.which when running on Bun, spawns `which` command otherwise.
+ */
+
+declare const Bun: typeof import("bun") | undefined;
+
+function which(command: string): string | null {
+  if (typeof Bun !== "undefined") {
+    return Bun.which(command);
+  }
+
+  // Fallback for Node.js or other runtimes
+  const { execSync } = require("node:child_process");
+  try {
+    const result = execSync(`which ${command}`, {
+      encoding: "utf-8",
+      stdio: ["ignore", "pipe", "ignore"],
+    });
+    return result.trim() || null;
+  } catch {
+    return null;
+  }
+}
+
+export default which;
+export { which };

test/js/bun/util/internal-which.test.ts

@@ -0,0 +1,27 @@
+import { expect, test } from "bun:test";
+import { which } from "bun:internal-for-testing";
+import { isWindows } from "harness";
+
+test("which finds executables in PATH", () => {
+  // "bun" should always be in PATH during tests
+  const bunPath = which("bun");
+  expect(bunPath).not.toBe(null);
+  expect(bunPath!.length).toBeGreaterThan(0);
+});
+
+test("which returns null for non-existent commands", () => {
+  const result = which("this-command-definitely-does-not-exist-12345");
+  expect(result).toBe(null);
+});
+
+if (!isWindows) {
+  test("which finds common system utilities", () => {
+    // At least one of these should exist on any POSIX system
+    const ls = which("ls");
+    const cat = which("cat");
+    const sh = which("sh");
+
+    const foundAtLeastOne = ls !== null || cat !== null || sh !== null;
+    expect(foundAtLeastOne).toBe(true);
+  });
+}