Terminal version updates: timeout() and os.spawn().

[orbitalquark]

I've pushed changes to the terminal version that should show up in the next nightly build:

• timeout() is now implemented for all platforms
• os.spawn() is now implemented for Windows

(These features were already available for the GUI versions on all platforms.)

In order to support os.spawn() on Windows, the terminal version of Textadept now uses the cross-platform reproc library for process I/O on all platforms. cmake --build should pick up the changes, pull in the dependency, and then build it into the terminal version. (The GUI version remains unaffected and uses Qt/Gtk's builtin process I/O capabilities.)

Please let me know if you run into issues and I'll see what I can do.