Add Promise-based versions for some functions in child_process

### What is the problem this feature will solve? It would be nice if child_process functionality, such as `exec` and `execFile`, had Promised based versions by default. Currently it does not, see https://nodejs.org/api/child_process.html Note that only child_process functions that can be async/await should be considered. This does not include things such as `fork` or `spawn`. ### What is the feature you are proposing to solve the problem? A new `child_process/promises` import path that can be used similar to how [fs promises](https://nodejs.org/api/fs.html#fs_promise_example) are used: ```js // Using ESM Module syntax: import { exec } from 'child_process/promises'; try { const { stdout } = await exec( 'sysctl -n net.ipv4.ip_local_port_range' ); console.log('successfully executed the child process command'); } catch (error) { console.error('there was an error:', error.message); } ``` The ask is basically that `child_process/promises` could just return a variant such as `const exec = util.promisify(process.exec);` as a convenience. ### What alternatives have you considered? I am already using `const exec = util.promisify(process.exec);` but that is not as nice. And this new proposal follows along what is happening in other Node.js APIs. (The original ask in https://github.com/nodejs/node/issues/38823 was perceived as asking for the entire module and all APIs to be async/await. This issue here narrows it down to only focus on the functions that can be.)