Bump league/commonmark from 2.6.0 to 2.8.0 (#5)

Bumps [league/commonmark](https://github.com/thephpleague/commonmark) from 2.6.0 to 2.8.0.
- [Release notes](https://github.com/thephpleague/commonmark/releases)
- [Changelog](https://github.com/thephpleague/commonmark/blob/2.8/CHANGELOG.md)
- [Commits](thephpleague/commonmark@2.6.0...2.8.0)

---
updated-dependencies:
- dependency-name: league/commonmark
  dependency-version: 2.8.0
  dependency-type: indirect
...

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