Add support for OSC7

[davidrg]

This escape is sent by some shells to let the terminal emulator know what the current active working directory is.

We can't directly do anything useful with this information as there is no guarantee Kermit 95 is running on the same machine as the shell (the only situation where this is possible is via PTY), but we could still store the information and make it available as a variable for use in scripts. Might be nice to automatically call a macro (perhaps ON_RCD) whenever the remote directory is changed to let the user do something with this information if they wish.

More information:

• https://gitlab.freedesktop.org/terminal-wg/specifications/-/issues/20
• ConEMUs similar 9;9 escape: https://conemu.github.io/en/AnsiEscapeCodes.html#ConEmu_specific_OSC