Support case-sensitive windows directories

[Gudahtt]

Checks for both upper-case and lower-case file extensions. This is to
support case-sensitive directories on Windows.

Mixed-case file extensions still won't work, but those are rare enough
that I don't think it's worth the performance cost to check those as
well.

Closes #57

[Gudahtt]

I didn't add tests for this fix, as I wasn't sure how. The per-directory case-sensitivity flag wasn't added until build 17093, which was fairly recent.

I tested it locally by editing the lines in test/basic.js where fixtures was deleted. I changed them to delete fixtures/foo.sh instead, leaving the directory. Then I created the fixtures directory and set the case-sensitivity flag on it using the command fsutil.exe file setCaseSensitiveInfo test/fixtures enable.

Once doing that, I added tests that would use process.env.PATHEXT (#58 ), and ran them. find when executable > with process.env.PATHEXT > foo was the failing test. It passed after making this change.

[Gudahtt]

I'm tempted to change this to use fs.readdir instead, but I'm not sure that would be a good idea. It would make this work for mixed-case file extensions, and would probably be faster for small directories. But for large directories (i.e. containing many files), this would be slower and would consume more memory.

[danielgary]

Are there any plans to merge this PR? This is a hangup on npx and yarn.

[wraithgar]

#100