fix microtask mode compatibility

[x04]

What does this PR do?

Fixes this issue: #20145

How did you verify your code works?

Has a test, requires using my patched WebKit so that will need to be merged before merging this.
See WebKit PR here: oven-sh/WebKit#101

[x04]

I noticed promise rejections are not handled properly in VMs either, but that is outside the scope of this PR.

If anyone wants to see the issue update the test script with this and run with node/bun:

const shouldThrow = process.argv[3];

// ...

let code = `
  Promise.resolve().then(() => {
    console.log('Microtask inside VM');
  });

  Promise.resolve().then(() => {
    console.log('Microtask inside VM 2');
  });

  console.log('End of VM code');
`;

if (shouldThrow === "true") {
  code = `
    Promise.resolve().then(() => {
      throw new Error('Test error');
    });

    console.log('End of VM code');
  `;
}

Node handles the rejection but Bun silently ignores it. I might fix that just not in this PR.