fix(ext/webidl): change createPromiseConverter

[marcosc90]

This commit fixes a few of the missing WPTs for Streams.

It also fixes the remaining two tests in #16276 to make it 39/39

[lucacasonato]

I don't understand how this differs from the existing code. PromiseResolve(V) already turns a thenable into a native Promise. Can you elaborate on what this is meant to do?

[marcosc90]

This change fixes the tests because the current logic (main) always adds an extra Promise by calling .then, see the following snippet

---

This patch:

function promiseConverter(p) {
  return Promise.resolve(p);
}

const p1 = new Promise(resolve => resolve()).then(() => console.log('1'));
const p2 = promiseConverter(undefined).then(() => console.log('2'));
const p3 = new Promise(resolve => resolve()).then(() => console.log('3'));
// 1, 2, 3

main

function promiseConverter(p) {
  return Promise.resolve(p).then(() => undefined);
}

const p1 = new Promise(resolve => resolve()).then(() => console.log('1'));
const p2 = promiseConverter(undefined).then(() => console.log('2'));
const p3 = new Promise(resolve => resolve()).then(() => console.log('3'));

// 1, 3, 2

So the code checks whether it's a thenable or not to avoid calling .then if it's not, and just call Promise.resolve