chore: create pre-commit file

[alexeagle]

Similar to https://github.com/bazelbuild/examples/pull/310 This ensures that contributors outside google3 have a means to easily make buildifier-clean commits. It is purely opt-in, and we assume that google3 users will have buildifier run for them in another way.

Other than the .pre-commit-config.yaml change, the rest is mechanically applied by buildifier.