breaking(layout_column_wrap): Make width optional but must be named

[gadenbuie]

Supersedes #799
Addresses posit-dev/py-shiny#732

Breaking: This PR makes width an optional argument and moves it to after the .... A code search shows that most people name the width argument, but in case they haven't we'll test to see if the first argument to layout_column_wrap() looks like a valid width value (a single numeric value or a valid CSS unit) and we'll emit a deprecation warning and use that first value as width.

Note: in the deprecation warning I'm assuming this breaking change will cause our next version to be v0.6.0 (in any case, we should review our deprecation warnings before the next release to make sure we have the right version in them).

Some code to try out this feature:

x <- lapply(1:3, \(x) card(lorem::ipsum(x)))

# `width` is no longer required
layout_column_wrap(!!!x)

# Explicit, named `width`
layout_column_wrap(width = 1/2, !!!x)
layout_column_wrap(width = "300px", !!!x)
layout_column_wrap(width = 400, !!!x)

# Implied as the first argument,
# leads to deprecation warning but gives same result
layout_column_wrap(1/2, !!!x)
layout_column_wrap("300px", !!!x)
layout_column_wrap(400, !!!x)

[cpsievert]

Nice, thanks!

I agree, the release should be tagged as v0.6.0.

After a quick look, I think we only need to update a deprecated_soft() call in value_box()?