Add ihtml.height for interactive custom widget height

[olivroy]

To mitigate #1544, specifying manually height could be useful.

Before, there was no way to prevent overflowing with opt_interactive(). Let me know if there is a better way to do this. I tried modifying the height value to NULL, or "auto" in the reactable::reactable() call in render_as_ihtml(), but couldn't find a way to do this well. The only way I found was with manual override. Figure a way to find out the RStudio viewer's size. But after my research, I figured that giving the option to user exactly how to set their widget's height was a good way to go?

I am not too familiar with setting options, and this will likely ignore static options set. I don't know which tab_options() are expected to have an effect in both interactive and non-interactive?

Before:

mtcars %>% gt() %>% opt_interactive()

This PR:

mtcars %>% gt() %>% opt_interactive(height = 300)

[rich-iannone]

This is actually ideal! There is container_height but it doesn't quite fit, and I think a scoped ihtml.* property is better here.

[rich-iannone]

LGTM!