Changes:
- Add WindowsPty, which uses the ConPTY API to create a pseudo console
- Pty now selects between PosixPty and WindowsPty
- Windows support in Command, including the ability to launch a process with a pseudo console
- Enable Command tests on windows
- Add some environment variable abstractions to handle the missing libc APIs on Windows
- Windows version of ReadThread
Makes progress getting "zig build test" to work on windows. Mostly
fixed issues around build configuration and added some branches throughout
the Zig code to return/throw errors for unimplemented parts.
I also added an initial implementation for getting the home dir.
