Add File/Dir formatters for mswin platform

[gromnitsky]

The mswin platform is Ruby build with the Visual Studio C++ compiler. Unlike msys2 or cygwin builds, it knows no unix features.

Without this patch, typing URI.open 'https://www.google.com' in irb throws an error, for AmazingPrint tries to shell out to ls(1) to get info about a tmp file & there is no ls executable on Windows.

This PL emulates powershell Get-ChildItem cmdlet output, e.g.:

> irb --prompt simple -r.\lib\amazing_print -ropen-uri
>> AmazingPrint.irb!
:output_value
>> URI.open 'https://www.google.com'
#<File:C:/Users/alex/AppData/Local/Temp/open-uri20200801-10928-k09w4s>
-a----       2020-08-01     22:58          14715 C:/Users/alex/AppData/Local/Temp/open-uri20200801-10928-k09w4s
>> Dir.open File.join ENV['USERPROFILE'], 'AppData'
#<Dir:C:\Users\alex/AppData>
d--h--       2019-06-18     18:00                C:\Users\alex/AppData

I added :unix & :mswin tags to several rspec examples, thus you can run rspec under Linux as before without noticing Windows-specific tests.

[HarlemSquirrel]

This looks good to me! Thank you for opening this and I apologize for not reviewing it earlier.

Please resolve the conflicts when you have a chance and we can merge this in 😊

[paddor]

Maybe you can provide two different implementations of #info according to RUBY_PLATFORM?
Like this:

if RUBY_PLATFORM.include?('mswin')
  def info
    # ...
  end
else
  def info
    # ...
  end
end

[paddor]

Maybe it makes sense to use the platform tester from RubyGems: Gem.win_platform?