Release v1.5.1

Based Prettier version: (2.8.0 →) 2.8.3

See the upstream change of 2.8.1‒2.8.3 especially for Markdown: https://github.com/prettier/prettier/blob/main/CHANGELOG.md#283

Important changes:

• Don't lowercase link references (prettier/prettier#13155 by @DerekNonGeneric & @fisker)
  https://github.com/prettier/prettier/blob/main/CHANGELOG.md#dont-lowercase-link-references-13155-by-dereknongeneric--fisker