Support parsing release notes from changelog files

As [Keep a Changelog](https://keepachangelog.com/) described, a good changelog should look like:

```md
## [Unreleased]

## [1.1.1]
### Added
- 1
- 2
### Fixed
- 3
- 4
### Changed
- 5
- 6

## [1.1.0]
...
```

We can do something like [ffurrer2/extract-release-notes](https://github.com/ffurrer2/extract-release-notes) to extract release notes from changelogs, that would be easier for GH CLI to create releases.