resolve
Adds relative file to this, considering this as a directory.
If relative has a root, relative is returned back.
For instance, File("/foo/bar").resolve(File("gav")) is File("/foo/bar/gav").
This function is complementary with relativeTo,
so f.resolve(g.relativeTo(f)) == g should be always true except for different roots case.
Return concatenated this and relative paths, or just relative if it's absolute.