Make `card_header()` display flex (equivalent to `.hstack`) by default

[gadenbuie]

Inspired by #1250 (comment), can we make .card_header have .hstack properties by default, with something like this:

.card-header {
  display: flex;
  flex-direction: row;
  align-items: center;
  align-self: stretch;
  gap: calc(var(--bslib-spacer, 1rem) / 2);
}

We'd also give card_header() a .bslib-gap-spacing by default. The goal is to enable toolbar-style card headers -- which is likely what most people want -- by default, e.g.

card(
  card_header(
    icon("gear"),
    p("Histogram of waiting times"),
    actionButton("go", "Go", class = "ms-auto btn-sm border-0")
  ),
  plotOutput(outputId = "distPlot"),
  card_footer(class = "text-muted", lorem::ipsum(2))
)

Note that we wouldn't do the same for card_footer() because it's more likely that these elements would take footnotes or prose.