Proper url even on windows

[visma-sorinbroscaru]

I noticed that on windows the url are actually rendered with \ instead of proper url /.
Test pass, as far as I can tell. ( On windows I had to adapt the tests expected strings to something account for platform separator.

expect(copyFileSyncMock).nthCalledWith(2, "test2.png", "dist/test2.png".replace(/\//g, sep)); //line 298 in smartAsset.test.js

in order to make it work. Since paths on windows are \\ instead of the / that is used in unix paths.

This fix should fix following scenarios:

//rollup.config.js
// ...
smartAsset({
    useHash: false,
    keepName: true,
    rebasePath: resolve(process.cwd(), 'node_modules'),
    publicPath: `/my/public/path/vendors`
}),
//...

FROM:

// compiled before changes
/* loaded by smart-asset */
var dottedBlue = "/my/public/path/vendors/@vismaux\\nordic-cool\\dist\\img\\spinners\\spinner_dotted_blue_light.svg";

TO:

// compiled after changes
/* loaded by smart-asset */
var dottedBlue = "/my/public/path/vendors/@vismaux/nordic-cool/dist/img/spinners/spinner_dotted_blue_light.svg";

Note: it was not really affecting the loading of the resource, however, I prefer universal url format style in this case.
Also note: As I looked into the copy mode, I realise that normalizeSlashes does nothing for it, however, if there will ever be the case where, just like @rollup/plugin-url, a [dirname] naming option will be included, the url should still look nice ( i hope so )