[Accessibility] Closing #679: Use <td> instead of <th> for title and subtitle

[jooyoungseo]

This PR addresses #679: now screen readers don't get confused when using title and subtitle element.

Reprex

library(gt)

# Create gt table with title and subtitle
airquality[1:10, ] %>%
  gt::gt() %>%
  gt::tab_header(
    title = "Title",
    subtitle = "Subtitle"
  )

Results

title and subtitle now get treated as <td>. This is good for screen reader a11y.

@rich-iannone, I still need your help to verify whether there are any CSS tweaks that we need to consider. I don't want to introduce any visual breaks.

[rich-iannone]

lgtm