Adds Changed header to buildpack version updates

[colincasey]

• will merge the updated buildpack versions with any existing Changed entries
• otherwise will create a new Changed header containing the updated buildpack versions

Fixes #130

[Malax]

I would've loved to see use working on the AST of the changelog instead of juggling strings. Especially since we already have a parsed representation for the changelog in the Changelog struct. We could extend it to include sub-headings. We could be more strict also since we know which sub-headings would be valid in the keep a changelog spec.

I will not block the PR on this (hence the approve). Please either rework the PR to level-up the Changelog struct or add an issue for it. :)

[colincasey]

@Malax I agree. My first iteration was an attempt to build out a more compliant structure for the Keep A Changelog spec but that's complicated by the fact that several of our existing changelogs do not comply with a stricter spec. E.g.;

• https://github.com/heroku/buildpacks-jvm/blob/main/buildpacks/jvm/CHANGELOG.md
• https://github.com/heroku/buildpacks-nodejs/blob/main/buildpacks/nodejs-corepack/CHANGELOG.md

To do this levelling-up we would need to first discuss the following:

• How should we deal with those existing non-compliant changelogs?
• How should we track the "No Changes" case which does easily fit into the spec?

I'll create an issue to track that discussion.

[colincasey]

Created #153