Support for running "Cygwin" (MinGW/MSYS) terminals on Windows

[afbjorklund]

Currently the posix code is compiled out, so it's not possible to run it from Windows.

But when isatty.IsCygwinTerminal, it should be preferred over the isatty.IsTerminal

This is the terminal:

https://mintty.github.io/

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The library might need some refactoring, to have access to both of the implementations ?

The simple code can be copy/paste'd, but that would look ugly for e.g. cursor and such

[afbjorklund]

Apparently a duplicate of #148, which used winpty.exe as a workaround

• https://github.com/AlecAivazis/survey/issues/148

It was needed for "lima", for running Linux machines (also on Windows)

• https://github.com/lima-vm/lima/issues/936

[AlecAivazis]

Closing this as a duplicate