v3.4.2: [Bugfix] Ensure writable streams are ended on PTY close (#59)

### What is this PR?

The writable stream in PTY is never set to 'unwritable' when PTY is
closed or when the child process dies.

### Changes

- Add `this.#writable.end()` in `close()` and `exit()` methods of Pty
class
- Update tests to verify write and read streams are no longer
writable/readable after PTY closes