Enable continuous (in addition to discrete) color scales

[pjireland]

One feature enhancement that would be useful for me is the ability to plot colors with continuous (in addition to discrete) color scales.

An example of where this could be useful is I have more bins than the number of colors allowed in a discrete color scale and do not want colors to repeat. With a continuous color scale, I could ensure the colors are adjusted based on my bins, regardless of the number of bins.

Continuous color scales are used in px.bar_plot, which Rosely uses for plotting, and so this should be possible to implement within Rosely. The keywords in px.bar_plot that would need to get supported in the Rosely interface are color_continuous_scale, range_color, and color_continuous_midpoint.