refactor(BREAKING): command pipe writes may now return a promise

[dsherret]

Before:

export interface CommandPipeWriter extends WriterSync {
  writeSync(p: Uint8Array): number;
  writeText(text: string): void;
  writeLine(text: string): void;
}

After:

export interface CommandPipeWriter {
  write(p: Uint8Array): Promise<number> | number;
  writeText(text: string): Promise<void> | void;
  writeLine(text: string): Promise<void> | void;
}

For performance, check if the write in a command returns a promise. If it does, then continue async, otherwise continue sync.

Closes #224