Add stub for filesystem storage backend

References: https://todo.sr.ht/~sircmpwn/tokidoki/2

storage/filesystem.go

@@ -0,0 +1,42 @@
+package storage
+
+import (
+	"github.com/emersion/go-vcard"
+	"github.com/emersion/go-webdav/carddav"
+)
+
+type filesystemBackend struct {
+	path string
+}
+
+var _ carddav.Backend = (*filesystemBackend)(nil)
+
+func NewfilesystemBackend(path string) carddav.Backend {
+	return &filesystemBackend{
+		path: path,
+	}
+}
+
+func (*filesystemBackend) AddressBook() (*carddav.AddressBook, error) {
+	panic("TODO")
+}
+
+func (*filesystemBackend) GetAddressObject(path string, req *carddav.AddressDataRequest) (*carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*filesystemBackend) ListAddressObjects(req *carddav.AddressDataRequest) ([]carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*filesystemBackend) QueryAddressObjects(query *carddav.AddressBookQuery) ([]carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*filesystemBackend) PutAddressObject(path string, card vcard.Card) (loc string, err error) {
+	panic("TODO")
+}
+
+func (*filesystemBackend) DeleteAddressObject(path string) error {
+	panic("TODO")
+}