Prettier ignore changeset's format (#79203)
### Why?
When adding changesets via the changeset/bot, it uses double quotes,
which fails lint. The lint is not really important for changesets, so
ignore running prettier for `.changeset/*.md`.
https://github.com/vercel/next.js/actions/runs/15020159472/job/42207301914?pr=79193#step:34:45
