On Windows, `ManifestToEpub` produces manifest and zip directory with backslash-separated paths

[cbutcosk]

Over at quire we've been tracking a bug with the use of epubjs-cli on Windows to package epub files. When packaging the epub manifest on windows, valid url-separator-paths in href properties on resources objects:

{
    "resources": [
        {
            "encodingFormat": "text/css",
            "url": "_assets/epub.css"
        },
        {
            "rel": "cover-image",
            "url": "_assets/images/static-cover.jpg"
        },
        {
            "url": "_assets/images/logo-getty.png"
        },
        {
            "url": "_assets/images/spiral-overlay.png"
        }
    ]
}

get Windows-style path separators when being packaged in to the package.opf file and when being inserted into the zip directory of the final .epub file:

    <item id="static-cover" href="_assets\images\static-cover.jpg" media-type="image/jpeg" properties="cover-image" />
    <item id="logo-getty" href="_assets\images\logo-getty.png" media-type="image/png" />
    <item id="spiral-overlay" href="_assets\images\spiral-overlay.png" media-type="image/png" /

I believe this is due to the usage of platform-specific path functions in epubjs-cli's ManifestToEpub. I've attached a reproduction test case (works in a VM as well) here:

epubjc-cli-test.zip