Add vfill-item CSS class when height isn't specified by user

[cpsievert]

With this PR, the htmlwidget's HTML container gains a .vfill-item CSS class when height is NULL (on the widget instance when statically rendered or shinyWidgetOutput() when rendered in shiny).

The primary motivation for this is so widgets can fill as needed inside bslib::card_body_fill() (#452); but more generally, it allows widgets (with height of NULL) to fill any HTML element with a .vfill-container class (when used with {bslib}, since those CSS classes are provided by bs_theme()), for example:

library(bslib)
library(htmltools)

browsable(page_fluid(
    theme = bs_theme(),
    style = "height:100vh", 
    class = "vfill-container",
    plotly::plot_ly()
))

This means, at least for widgets that follow the recommended R binding approach, statically rendered widgets (such as above) won't need any changes to fill by default (within a vfill-container). However, the recommend Shiny binding approach of

sigmaOutput <- function(outputId, width = "100%", height = "400px") {
  shinyWidgetOutput(outputId, "sigma", width, height, package = "sigma")
}

will change to

sigmaOutput <- function(outputId, width = NULL, height = NULL) {
  shinyWidgetOutput(outputId, "sigma", width, height, package = "sigma")
}

and default sizing should instead be provided through a CSS file, providing something like:

.sigma.html-widget {
  width: 100%;
  height: 400px;
}

TODO

Update website (and the sigma package?) to reflect the new default sizing approach

[cpsievert]

Closing in favor of #442