Support `Seek()` in `fs.FS` runfiles implementation

[mering]

Currently the fs.FS.Open() call returns an fs.File interface which does provide the .Seek() method. Return an interface inheriting from both fs.File and io.Seeker and provide the .Seek() method in the renamedFileInfo struct in fs.go.

Note: This approach is also used by the core http package, see golang/go#66877.
Example: https://gotipplay.golang.org/p/o9ABZj7Sfx6