chore: introduce buildifier format/linter for starlark

[alexeagle]

So far this is just opt-in, you install from pre-commit.com and run 'pre-commit install' to get your commits formatted nicely. We can add some CI enforcement for this in a follow-up PR.

Ran 'pre-commit run --all-files' to do an initial pass over the repo and made it warning-clean.

[p0deje]

We can add some CI enforcement for this in a follow-up PR.

Can we have a Bazel test target to do so? This would make it much easier.

[alexeagle]

Can we have a Bazel test target to do so?

No, this is one of those cases where

• we want to check the entire repo, and it's a bad antipattern to have any target that "depends on all X"
• some of the files we want to format/lint are not part of any target - it would be annoying to be forced to create some filegroup or something that has BUILD.bazel files as its srcs

I'll send a PR now to add the https://github.com/bazel-contrib/rules-template/blob/main/.github/workflows/buildifier.yaml file