update-readme.mjs generates different content on different os

[JasonYeMSFT]

update-readme.mjs generates content that contains bundled files for each skill. Skills can have nested folders/files. Currently it generates file paths with / as delimiter on non-Windows platforms and file paths with \ as delimiter on Windows due to how path.join works. From what I can tell, a CI task checks the diff of the committed readme content vs. its locally generated readme content which will fail if the user generated the readme content on Windows. Idealy the script should generate platform agnostic content which will also make cross platform collaboration easier.