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.

swift/extractor/BUILD.bazel

@@ -1,5 +1,5 @@
-load("//swift:rules.bzl", "swift_cc_binary")
 load("//misc/bazel:pkg_runfiles.bzl", "pkg_runfiles")
+load("//swift:rules.bzl", "swift_cc_binary")
 
 swift_cc_binary(
     name = "extractor.real",
@@ -7,6 +7,10 @@ swift_cc_binary(
         "*.h",
         "*.cpp",
     ]),
+    linkopts = select({
+        "@platforms//os:macos": ["-headerpad_max_install_names"],
+        "//conditions:default": [],
+    }),
     visibility = ["//swift:__pkg__"],
     deps = [
         "//swift/extractor/config",
@@ -17,10 +21,6 @@ swift_cc_binary(
         "//swift/third_party/swift-llvm-support",
         "@absl//absl/strings",
     ],
-    linkopts = select({
-        "@platforms//os:macos": ["-headerpad_max_install_names"],
-        "//conditions:default": [],
-    }),
 )
 
 sh_binary(