fix: Implement some logic for isExecutable for windows

[arran4]

This is based on:

• https://www.nextofwindows.com/what-is-pathext-environment-variable-in-windows
• It goes back to at least Windows XP: https://ss64.com/nt/syntax-variables.html potentially further

fix: Provide some logic for isExecutable under windows for determining if it's an executable

BREAKING CHANGE: isExecutable no longer always returns false and tries to determine execution status by file extension

[halostatue]

This looks good to me. I’m going to defer approval to @twpayne because it is a breaking change, but I think that it’s a sensible change. I do not expect that anything is depending on isExecutable returning false on Windows, but Hyrum’s law always applies.

[arran4]

I think it is primarily used for one of the source control systems. In which case it might need to be split.

Defining the correct level of abstraction is a completely separate discussion (see Mythical Man-Month)

I don't remember that being in the book, but means I can refer to it a lot more. The 70s man..

[twpayne]

Thanks, this makes sense. There are few minor changes I'd like to do to make the control flow more compact, but I'll do this in a separate PR.

[arran4]

Thanks I will read.