Add extended-length prefix support

[nkaretnikov]

Fixes #588.

Description

This pull request:

• adds support for the Windows extended-length prefix ("\\?\") via the win_extended_length_prefix option
• adds a test
• updates the docs.

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

Additional information

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[nkaretnikov]

@jaimergp PTAL. I'll rebase after you approve since a rebase will make it harder to review the recent changes, which is just a fixture update.