Close non-std FDs between pty_forkpty and pty_execvpe

[htfy96]

Environment details

• OS: Linux
• OS version: Linux 6.7
• node-pty version: The one bundled in VSCode, and I believe it still exists on master

Issue description

Related to microsoft/vscode#202558 .

Currently node-pty doesn't close unwanted fds before spawning the process in pty_execvpe. This caused the above issue where Node's open fd(s) accidentally leaks into the spawned process. Considering that very few spawned shell process are aware to the non-std fds, I doubt if this would ever be the desired behavior.

To reduce the amount of surprise, I hope it could do something like https://github.com/facebook/folly/blob/main/folly/Subprocess.cpp#L559 to close all unwanted fds before spawning the shell.