Awaiting onDone() too late causes execution to hang

[legraphista]

Due to how onDone has been implemented, calling it after the process is finished will result in a hanged promise:

ffmpeg-progress-wrapper/src/index.ts

Lines 216 to 220 in b2ea41e

	const { code, signal } = await new Promise<{ code: number, signal: string }>((res) => {
	this.once('end', (code: number, signal: string) => {
	return res({ code, signal })
	})
	});

The issue stems from the fact that once the sub-process has ended, the end event will not fire again, thus locking the promise in an unresolvable state