Build: Force LF for JS source files

This takes exactly the same approach as this: https://github.com/jzaefferer/jquery-validation/blob/9c83cac3ed9ecb02557133d55b6dca616042e192/.gitattributes

@nschonni addressed the issue he had himself and there were no reported issues since then. With `* text=auto` we still ensure that all text files in the repo use LF, but we only force JS files to always use `LF` for our tools to work. We don't need that for any other files.

This replaces #1196.

@fnagel @gnarf can you give this one a try?