Split PAGER and EDITOR with a Windows-specific shlex equivalent

[kdeldycke]

When I implemented #3777 a couple of weeks ago, I was annoyed by Windows behavior. And wondered why shlex.split is only available on Unix-like platforms. Then while cleaning ancient Python 2 stuff in https://github.com/pallets/click/pull/3817/changes#diff-3bd2075eb8cab59a601f5a021078e5763eea117ae315b04d8d2d719de1f45a97R48 , I discovered CommandLineToArgvW.

This is an attempt to rely on CommandLineToArgvW, which seems to do the trick, as a Windows equivalent to shlex.split.

But digging deeper I stumbled upon the discussion upstream at python/cpython#44990 (comment) , which explain why it was not considered for inclusion in the Python stdlib:

on Windows, there isn't a single "shell syntax" to target - PowerShell and CMD have very different quoting rules, etc, and the fact that commands can do their own command line parsing (they get passed a command line string rather than an argument array) adds even more complexity.

And that explain why this PRs has limits, and why I choose to keep the shell completion relying on shlex.

I propose this PR anyway as it seems to bring the behavior more in line with Windows default, but kept as draft as I need a bit of feedback about this attempt.

Tests

I collected test cases from past discussions to build up a more comprehensive suite:

• Can't edit file with " quotation mark in name #1760
• Use subprocess instead of os.system #1477
• Extra quotation mark appended to string arguments with escaped or quoted spaces and a trailing backslash #2486