Tested only on windows with a USB serial connector to my NUC running
linux.
This allows opening up wezterm on a serial port connection.
Serial is closer to a tty than a pty, so it is a bit different
to configure and use.
This commit allows running:
```
wezterm serial COM3
```
to open a window that connects to the specified COM port.
You can change the baud rate with:
```
wezterm serial --baud 9600 COM3
```
There are more options that could be set, but I'm a bit lazy and
have only exposed the baud rate to the wezterm cli so far.
This commit adds (untested!) support for remote ttys over ssh.
It requires that the embedding application connect and authenticate
the ssh session prior to turning it into a PtySystem implementation.
