Allow a transform's process function to return Promise

[a-student]

🚀 Feature Proposal

Jest expects an exported transform's process function to return a string.
Please, allow it to return a Promise<string | { code: string }> | string | { code: string }.

Motivation

We live in a new era of asynchronous code and, unfortunately, it is not always possible to write synchronous transformations. For example, I implement a transform which uses postcss internally, which in turn is asynchronous all the way. Currently, I unable to use postcss as a dependency for transformations :-(

Example

Somewhere in a transform implementation:

export async function process (
    src: string,
    path: string,
    config: any,
): Promise<string> {

... await ...

    return '';
}

[SimenB]

This is not possible as transformers are called at require time, which is synchronous.

Duplicate of #5556 et al

[github-actions]

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