Add fs.exists[util.promisify.custom] (#8936)

* Add fs.exists[util.promisify.custom]

fs.exists doesn't follow the error-first-callback convention, so it
needs a custom implementation for util.promisify.

* Simplify

---------

Co-authored-by: John-David Dalton <john.david.dalton@gmail.com>

src/js/node/fs.js

@@ -365,6 +365,8 @@ var access = function access(...args) {
 // TODO: make symbols a separate export somewhere
 var kCustomPromisifiedSymbol = Symbol.for("nodejs.util.promisify.custom");
 
+exists[kCustomPromisifiedSymbol] = path => new Promise(resolve => exists(path, resolve));
+
 read[kCustomPromisifiedSymbol] = async function (fd, bufferOrOptions, ...rest) {
   const { isArrayBufferView } = require("node:util/types");
   let buffer;

test/js/node/fs/fs.test.ts

@@ -1310,6 +1310,14 @@ describe("fs.exists", () => {
       }
     });
   });
+  it("should work with util.promisify when path exists", async () => {
+    const fsexists = promisify(fs.exists);
+    expect(await fsexists(import.meta.path)).toBe(true);
+  });
+  it("should work with util.promisify when path doesn't exist", async () => {
+    const fsexists = promisify(fs.exists);
+    expect(await fsexists(`${tmpdir()}/test-fs-exists-${Date.now()}`)).toBe(false);
+  });
 });
 
 describe("rm", () => {