Don't convert backslashes

not sure why it was added in the first place but it seems to be causing problems here:
mifi/lossless-cut#1941

e.g. on macos when trying to encode a file name with a backslash in it, it causes the backslashe to interpreted as forward slashes, which is not correct and causes the process opening the file to fail

index.js

@@ -12,8 +12,6 @@ export default function fileUrl(filePath, options = {}) {
 		pathName = path.resolve(filePath);
 	}
 
-	pathName = pathName.replace(/\\/g, '/');
-
 	// Windows drive letter must be prefixed with a slash.
 	if (pathName[0] !== '/') {
 		pathName = `/${pathName}`;