Plot brushing API in Express

[gshotwell]

Currently the brush options are defined in ui.output_plot() so it doesn't seem like plot brushing can be used in Express.

 ui.output_plot("plot1", click=True, dblclick=True, hover=True, brush=True)
 

Maybe we should pass these options through @render.plot() instead so they work in express.

Related to #975.

[cpsievert]

In case anyone is interesting in doing this today, it's possible with shiny.express.output_args, but it seems likely we'll come up with a better API. Here's some examples posit-dev/shinylive#108