fix: extraction of Mac packed asar files from Windows when the pack contains links

[Spacetech]

Fixes #50

If an asar is packed on Mac and it contains links across nested directories, the following error will occur when extracting the asar from Windows:

D:\Programming\asar\lib\filesystem.js:141
      return node.files[name]
                  ^

TypeError: Cannot read properties of undefined (reading 'files')
    at Filesystem.getNode (D:\Programming\asar\lib\filesystem.js:141:19)
    at Filesystem.getFile (D:\Programming\asar\lib\filesystem.js:149:23)
    at Filesystem.getFile (D:\Programming\asar\lib\filesystem.js:153:19)
    at Object.module.exports.extractAll (D:\Programming\asar\lib\asar.js:187:29)
    at Command.<anonymous> (D:\Programming\asar\bin\asar.js:72:10)
    at Command.listener [as _actionHandler] (D:\Programming\asar\node_modules\commander\index.js:408:31)
    at Command._parseCommand (D:\Programming\asar\node_modules\commander\index.js:909:14)
    at Command._dispatchSubcommand (D:\Programming\asar\node_modules\commander\index.js:860:18)
    at Command._parseCommand (D:\Programming\asar\node_modules\commander\index.js:877:12)
    at Command.parse (D:\Programming\asar\node_modules\commander\index.js:712:10)

      at ChildProcess.exithandler (child_process.js:308:12)
      at maybeClose (internal/child_process.js:998:16)
      at Process.ChildProcess._handle.onexit (internal/child_process.js:265:5)

It's caused by the path separator being different between Mac and Windows. getNode will now detect this case and fix the slashes.

I also added a test for this. extractthis-mac-links.asar was created on Mac and it contains the link dir2/dir4/file4_link.txt -> dir1/dir3/file4.txt. The test would fail when running on Windows without the new logic in getNode.

[Spacetech]

The CI pipeline appears to be broken on linux. It's unrelated to this change.