make the `._has_subplots` method from `go.Figure` accessible

In some cases, when writings callbacks that only update a subplot of a Figure, it is useful to know if the figure will accept an append_trace to a specific location, or if add_trace should be used instead.

I use this workaround (using a try/except isn't great):

    if fig._has_subplots():
        fig.append_trace(trace, 1, 1)
    else:
        _ = fig.add_trace(trace)

If I only use fig.append_trace(trace, 1, 1), I get Exception: In order to reference traces by row and column, you must first use plotly.tools.make_subplots to create the figure with a subplot grid.

If I only use fig.add_trace(trace), the layout gets messed up.

Could it be possible to make this method public?

Inspect go.Figure's _has_subplots implementation and the public Figure API around append_trace, add_trace, and make_subplots. The work is done when callers can use a public check to determine whether row-and-column trace references are valid without relying on a try/except workaround, while preserving existing subplot behavior.