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.

javascript/BUILD.bazel

@@ -1,5 +1,5 @@
-load("@semmle_code//:dist.bzl", "dist")
 load("@rules_pkg//pkg:mappings.bzl", "pkg_files")
+load("@semmle_code//:dist.bzl", "dist")
 load("@semmle_code//buildutils-internal:zipmerge.bzl", "zipmerge")
 
 package(default_visibility = ["//visibility:public"])

javascript/extractor/BUILD.bazel

@@ -1,5 +1,5 @@
-load("@semmle_code//:common.bzl", "codeql_fat_jar", "codeql_java_project")
 load("@rules_pkg//pkg:mappings.bzl", "pkg_files")
+load("@semmle_code//:common.bzl", "codeql_fat_jar", "codeql_java_project")
 
 java_library(
     name = "deps",