[Feature Request] fsspec/universal_pathlib integration

As far as I understand it, this package cannot be used with universal pathlib right away, but all the building block are already there. All that's missing is registering the `AbstractFileSystem` subclasses with fsspec and creating fsspec/universal_pathlib subclasses and registering them with their registry.

This seems simple enough and seems to work in my testing:

```python
from fsspec import AbstractFileSystem
from upath import UPath
from upath.registry import register_implementation

from morefs.memory import MemFS

# local instance for inspectability during testing
mem_fs = MemFS()

class MorefsMemPath(UPath):
@classmethod
def _fs_factory(
cls, urlpath: str, protocol: str, storage_options: dict
) -> AbstractFileSystem:
return mem_fs # this should be MemFS(...)

fsname = "customfs"
register_implementation(fsname, MorefsMemPath)

def test_upath():
testtext = "Hello from MemFS!"
testfile = "/example.txt"
p = UPath(f"{fsname}:///{testfile}")
p.write_text(testtext)
assert testtext == p.read_text()
assert mem_fs.ls("/") == [testfile]
```