Required for #44712

We started using `grid` for Markdown tables instead of flex. This
resulted in tables having a width of 0 inside popovers, since popovers
are laid out using `AvailableSpace::MinContent`.

One way to fix this is to lay out popovers using `MaxContent` instead.
But that would affect all Markdown rendered in popovers and could change
how popovers look, or regress things.

The other option is to fix it where the problem actually is:
`repeat(count, vec![minmax(length(0.0), fr(1.0))])`. Since the minimum
width here is `0`, laying things out with `MinContent` causes the
Markdown table to shrink completely. What we want instead is for the
minimum width to be the min-content size, but only for Markdown rendered
inside popovers.

This PR does exactly that, without interfering with the `grid_cols` API,
which intentionally follows a TailwindCSS-like convention. See
#44368 for context.

Release Notes:

- N/A