Parse and expose changelog

[IchordeDionysos]

Some package ecosystems expose less information out-of-the-box (e.g. Dart exposes changelogs automatically, while NPM does not).

It would be cool if deps.dev would write an abstract parser for this information that takes the information from various possible places (GitHub releases, CHANGELOG.md files, ...) and exposes it for all ecosystems (briding the gap for ones that don't have that)