Support factors in fmt_markdown() with HTML output

[rossellhayes]

Summary

This PR closes #1882 by ensuring factors are converted to text before converting to markdown when fmt_markdown() is used with HTML output. It achieves this by adding text <- as.character(text) as the first step in the md() function. It also adds a test of rendering a factor column with markdown formatting and updates the NEWS file.

gt(data.frame(x = factor("TEST"))) |> 
  fmt_markdown()

x
TEST

^{Created on 2024-09-17 with reprex v2.1.0}

Related GitHub Issues and PRs

• Ref: Using fmt_markdown() on a factor prints the numeric factor level when the output format is HTML #1882

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[rich-iannone]

This is great, thanks!