Remove the need for `fillable=False` in Shiny

[mwouts]

This is a follow-up on #360 and posit-dev/py-shinywidgets#199 - ideally we shouldn't need to resort to fillable=False.

[mwouts]

As mentioned by @cpsievert in this comment, DT has a fillContainer argument that makes it fit the container size - see https://rstudio.github.io/bslib/articles/filling/index.html#dt-tables