Bazel: introduce buildifier formatting

This introduces tooling and enforcement for formatting bazel files.

The tooling is provided as a bazel run target from
[keith/buildifier-prebuilt](https://github.com/keith/buildifier-prebuilt).

This is used in a [`pre-commit`](https://pre-commit.com/) hook for those
having that installed. In turn this is used in a CI check. Relying on a
`pre-commit` action gives us easy checking that buildifying did not
change anything in the files and printing the diff, without having to
hand-roll the check ourselves.

This enforcement will make usage of gazelle easier, as gazelle itself
might reformat files, even outside of `go`. Having them properly
formatted will allow gazelle to leave them unchanged, without needing
to configure awkward exclude directives.

BUILD.bazel

@@ -0,0 +1,9 @@
+load("@buildifier_prebuilt//:rules.bzl", "buildifier")
+
+buildifier(
+    name = "buildifier",
+    exclude_patterns = [
+        "./.git/*",
+    ],
+    lint_mode = "fix",
+)