fs: Adds tests to prove we allow fs.FS to create new files

[codefromthecrypt]

This ensures those who cannot use writefs.DirFS can use a fs.FS implementation that creates or opens files as read/write. This does so by making a hacked fs.FS and testing it.

type hackFS string

func (dir hackFS) Open(name string) (fs.File, error) {
	path := ensureTrailingPathSeparator(string(dir)) + name

	if f, err := os.OpenFile(path, os.O_RDWR, 0); err == nil {
		return f, nil
	} else if errors.Is(err, syscall.EISDIR) {
		return os.OpenFile(path, os.O_RDONLY, 0)
	} else {
		return nil, err
	}
}

This also removes type filtering to ensure hacked filesystems aren't accidentally masked. Instead we get more specific on error codes returned by functions such as os.File.Write.

---

Note: This is meant to extend the life of fs.FS for custom implementations, though we expect eventually to have our own API. However, such an API will remain internal until at least the end of the month due to a lot of prerequisite work. See #1013 cc @wjam

[codefromthecrypt]

instead of masking here, we are verifying the errno on Write. seems to work e.g. flags are honored to prohibit writes.

[codefromthecrypt]

this was moved as it is only used here now