Open Source trunk itself

[gewenyu99]

By Christian Höltje, moved from Canny

I think the trunk check, action, and fmt will move faster if the CLI tool is open-sourced.
I would have contributed some fixes and features if the source was available to contribute to.

[gewenyu99]

Team notes: this is under discussion. We're not completely opposed to this and we'd love to hear more input on this topic from the community.

For now, https://github.com/trunk-io/plugins and https://github.com/trunk-io/docs are open to contributors.