fix: allow cells_stub() to target all stub components

[rich-iannone]

This PR fixes an issue where stub components weren't all being targeted (i.e., in a multicolumn stub). Now, this example works properly:

exibble |>
  gt(
    rowname_col = c("group", "char")
  )|>
  tab_style(
    style = list(
      cell_fill(color = "lightblue")
    ),
    locations = list(cells_stub(columns = NULL))
  )

Previously, only the left column of the two-column stub was styled.

Fixes: #2100