Add fs.exists[util.promisify.custom]

[dmitri-gb]

What does this PR do?

This adds the missing implementation for fs.exists[util.promisify.custom].

fs.exists doesn't follow the error-first-callback convention, so it needs a custom implementation for util.promisify. Without it, a promisifed exists will always reject (with true or false as the reason).

Prisma CLI relies on this.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

I wrote automated tests.

• I included a test for the new code, or existing tests cover it
• I ran my tests locally and they pass (bun-debug test test-file-name.test)

[jdalton]

Relavant Node.js code:
https://github.com/nodejs/node/blob/53aed8838cb71142ca1591b655000aac90d91205/lib/fs.js#L276-L281

ObjectDefineProperty(exists, kCustomPromisifiedSymbol, {
  __proto__: null,
  value: function exists(path) { // eslint-disable-line func-name-matching
    return new Promise((resolve) => fs.exists(path, resolve));
  },
});

💡 Might be implemented this way in Node because of its older perf history with await.
Of interest is @Jarred-Sumner's twitter thread on JSC await perf improvements.

Node's doesn't suppress the in the promise form because it's already suppressing errors in fs.exists (as do we):
https://github.com/nodejs/node/blob/53aed8838cb71142ca1591b655000aac90d91205/lib/fs.js#L265-L273

[dmitri-gb]

@jdalton are you saying I should remove the try-catch? I only added it because I'm not really sure about the semantics of Bun.fs().exists (whether it ever throws/rejects etc) and also because the callback variant defintion adds a rejection handler:

fs.exists.$apply(fs, [path]).then(
  existed => callback(existed),
  _ => callback(false),
);

[jdalton]

That bun is handling the rejection and returning false aligns with expectations. The try-catch it's wrapped in helps for any synchronous issues so both(sync/async) are covered. No need to wrap it in additional try-catches. I'd just tweak to do as node does. It's a single line and keeps things simple:

new Promise((resolve) => fs.exists(path, resolve))

in a non async function.