This change allows for private watch methods. By convention they start with
an underscore. If a reactive or var has a private watch method, it will be
used in preference to a public watch method.
With this change it becomes easier to have a private reactive/var whose
watcher is also private. For example:
_counter = var(0)
"""This is a private counter, it won't appear in the docs."
...
def _watch__counter(self) -> None:
"""Watch _counter, but don't appear in the docs either."
...
See #2382.
