Add "xtask changelog ci" to validate CHANGELOG.md files

[exzachlyvv]

End goal (bug): Create script to automate updating CHANGELOG.md

This task will be done in 2 parts:

1. xtask changelog ci: validates all CHANGELOG.md files <-- this PR
2. xtask changelog {major,minor,patch}: automatically updates changelogs with a new description.

[ia0]

Thanks! I've created a dev/changelog branch for this work (making it the base branch). We can merge it at 2 different points in time:

• Once when we have the CI check (e.g. xtask changelog ci).
• Once when we have xtask changelog {major,minor,patch} commands.

[exzachlyvv]

Last part is still WIP - refactor string parsing to be stateful instead of rough sting manipulation.

[exzachlyvv]

This PR is ready for another pass. There is 1 validation missing: "The first version matches the Cargo.toml version." Will do that in follow up PR.

[ia0]

Thanks! Looks pretty good to me.

[ia0]

Thanks! I've updated the branch to master and added a small commit with nits. I didn't review the changelog.rs file. I'll do that in the follow-up PRs. Also, we should have a PR before then that moves the module from xtask to cli-tools.