[create-next-app]: add mdx files to tailwind content config (#48188)
This PR updates create-next-app's tailwind config to consider .mdx files
when it generates styles.
If you're using MDX, most of a project's styles are generated by the
other file flags, which can make it tricky to debug/notice the rare
subset from .mdx only.
x-ref: https://vercel.slack.com/archives/C0290CZ3U6Q/p1680976472444189
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Co-authored-by: JJ Kasper <jj@jjsweb.site>