feat(fs, config): add support for custom filesystem type construction (#9887)

For Synctrain I would like to create a virtual filesystem that exposes
iOS' photo library. This can only be accessed through APIs.

lib/fs/basicfs.go

@@ -19,6 +19,8 @@ import (
 	"github.com/syncthing/syncthing/lib/build"
 )
 
+const FilesystemTypeBasic FilesystemType = "basic"
+
 var (
 	errInvalidFilenameEmpty               = errors.New("name is invalid, must not be empty")
 	errInvalidFilenameWindowsSpacePeriod  = errors.New("name is invalid, must not end in space or period on Windows")
@@ -56,6 +58,12 @@ type (
 	groupCache = valueCache[string, *user.Group]
 )
 
+func init() {
+	RegisterFilesystemType(FilesystemTypeBasic, func(root string, opts ...Option) (Filesystem, error) {
+		return newBasicFilesystem(root, opts...), nil
+	})
+}
+
 func newBasicFilesystem(root string, opts ...Option) *BasicFilesystem {
 	if root == "" {
 		root = "." // Otherwise "" becomes "/" below