types: $concatArrays with $cond

[Marsup]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.3

Node.js version

16

MongoDB server version

N/A

Description

Very similarly to #12017, ConcatArrays doesn't allow $cond

Steps to Reproduce

import type { Expression } from 'mongoose';

const concat: Expression.ConcatArrays = {
    $concatArrays: [
        {
            $cond: {
                if: { $eq: ['foo', true] },
                then: [1],
                else: [2]
            }
        }
    ]
}

Playground link: https://www.typescriptlang.org/play?ssl=13&ssc=2&pln=1&pc=1#code/JYWwDg9gTgLgBDAnmApnA3nAogDzFFAZ0OAgDs4BfOAMyghDgHIRyBzCCQlJgbgCh+AY3KF4IskICGMAFzY8BYqTIA6AMLlpMAIJQoUxITgBeDPziW4AEgna9Bo-IDaFq+-Rv33mxIAm8p4+wVbANIE2KACOLkw0nEwANAhQAK4oALpUiV4h3jAAFihkLgCMGTl5wSgANtwuAEwZuSGULVZt7s2UQA

Expected Behavior

It should build without error