Add the equivalent of `bslib::layout_columns()`

[cpsievert]

Fairly recently, {bslib} gained layout_columns(), which is now our recommended starting point for column-wise layout.

https://rstudio.github.io/bslib/reference/index.html#multiple-columns

It also supersedes the the row()/column() interface (it supports filling layouts, wrapping onto new rows, etc), so when this gets added we should also drop row()/column() from express.

[cpsievert]

Once this gets added, I think there is a strong argument for removing layout_column_wrap(), row(), and column() from shiny.express.layout (and promoting this function as the layout "work-horse")

[gadenbuie]

I think we should keep layout_column_wrap(), just renamed as layout_columns_auto(), in Shiny Express. Both are useful and cover similar but distinct layouts. It will also be nice that you can change the suffix to switch between them as needed.

[gadenbuie]

Another option discussed today:

• layout_columns() for bslib::layout_columns()
  • Reason: it's a workhorse function that covers most of your column-layout needs)
• layout_columns_grid() for layout_column_wrap()
  • Reason: the result is a grid, i.e. a literal grid and not just a CSS grid, a.k.a. what most people think of when they think of "grid".