Add %{first_conventional_commit} variable for squash template
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<summary>I heard you like squashes...</summary>
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<small>Photo by <a href="https://unsplash.com/@nutsycoco?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText">Melanie Hughes</a> on <a href="https://unsplash.com/s/photos/squash?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText">Unsplash</a></small>
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## Description
[Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) is a convention for writing git commit messages which is used in a number of automation tasks, from compiling changelogs to cutting new releases. We use this convention when writing commit messages for `gitlab-ui` ([see relevant docs](https://gitlab.com/gitlab-org/gitlab-ui/-/blob/main/doc/contributing/commits.md#gitlab-uis-commit-conventions)).
Let's add a variable for `%{first_conventional_commit}` in the squash commit message template.
## Thoughts
- **What if there are more than one conventional commit?** That's a really good question but a bit out of scope. Squashing implies we only want 1 commit at the end of it. To properly solve this, we'll need to set up something like an interactive rebase through the UI.
- **What if there are no conventional commits?** I guess this would be the same behavior as `%{first_multiline_commit}` when there are no multiline commits?
issue