fix: keep isinstance(fig, go.Figure) working after register_plotly_resampler

[raashish1601]

Fixes #364

register_plotly_resampler() replaced go.Figure and go.FigureWidget with plain functions. That made isinstance(fig, go.Figure) raise TypeError: isinstance() arg 2 must be a type..., which breaks any downstream code that checks figure types.

The wrappers are now small classes instead of functions. Calling one still returns a FigureResampler / FigureWidgetResampler exactly as before (__new__ builds it, so __init__ never runs). Their metaclass forwards isinstance, issubclass and class attribute lookups to the original plotly class. So after registering, isinstance(fig, go.Figure) is True, issubclass(FigureResampler, go.Figure) is True, and go.Figure.<attr> still works like it did with the functools.wraps function. Unregistering and _get_plotly_constr work the same way through __wrapped__.

Tests: added test_isinstance_when_registered to tests/test_registering.py. It covers figure and widget mode, issubclass, and checks after unregistering. The existing tests asserted that the registered constructors are functions (inspect.isfunction), so they now use _already_wrapped instead. test_registering.py, test_composability.py and test_plotly_express.py pass. In test_serialization.py, the three FigureWidget copy tests fail locally both with and without this change, because my env has ipywidgets 8 (Widgets cannot be copied) and the project pins 7. ruff check and black --check are clean.

I also added a line to the "Latest" section of the changelog.