fix: standardize usage of /dev/tty where possible

[aschey]

Resolves #919
Resolves #396
Supersedes #743

Previous attempts to prefer /dev/tty over stdin had issues because of problems with polling /dev/tty on MacOS. This has been solved for a while with the use-dev-tty feature, so now it should be safe to prefer /dev/tty everywhere except for the mio event reader on MacOS.

This PR prioritizes the use of /dev/tty for both input and output by splitting the tty_fd function into tty_fd_in and tty_fd_out so both can handle the appropriate fallback logic for stdin and stdout respectively. The only place that needs special care is the MacOS event reader when use-dev-tty is not enabled.

The mio event reader for MacOS has been modified to return an explicit error message when used with piped input to inform users that use-dev-tty is required for such use cases. The error message was previously getting swallowed, but will now be returned to the caller.

Finally, the logic to read the cursor position was always writing to stdout, causing an error when stdout was not a tty. This has been changed to use the new tty_fd_out function.

This was tested on Mac, Linux, and WSL. Windows should be unaffected.