V4 Update add_trace, add_traces, and add_{trace} methods to return figure

[jonmmease]

This PR updates the add_trace, add_traces, and add_{trace} figure method to return the figure their called on, rather than the newly created trace(s).

This is a breaking change for v4, as these methods used to return the newly created traces. The motivation for the change is to support method chaining. e.g.

(make_subplots(rows=2)
 .add_scatter(y=[2, 1, 3], row=1, col=1)
 .add_bar(y=[2, 1, 3], row=2, col=1)
 .update_layout(title_text='Figure Title', template='plotly_white'))

Code that formerly relied on the returned trace

bar = fig.add_bar(...)

will need to be updated for v4 compatibility to grab the newly added trace from the returned figure's data array

bar = fig.add_bar(...).data[-1]

@nicolaskruchten