Function record_for

record_for(duration: number, buffer: number): SoundPromise
Records a sound of a given duration. Returns a Sound promise.

How the function behaves in detail:
1. record_for is called.
2. The function waits for the given buffer duration.
3. The recording signal is played.
4. Recording begins when the recording signal finishes.
5. The recording signal plays to signal the end after the given duration.
Parameters
- duration: number
  duration in seconds
- buffer: number
  pause before recording, in seconds
Returns SoundPromise
Example
```
init_record();
const promise = record_for(2, 0.5); // begin recording after 0.5s for 2s
const sound = promise();            // retrieve the recorded sound
play(sound);                        // and do whatever with it
```