Introduces treefmt with a simple nixfmt-rfc-style configuration to
format all Nix files.
This is only practically usable with the following commit that formats
all files accordingly.
Also post a comment in case base branch is wrong
This guides newcomers in how to smoothly handle the potentially scary
situation of having thousands of commits listed in a PR.
While CI shows the same, people might not even look at CI if the PR
looks botched.