Open .BUILD files in bazel-build-mode

[eklitzke]

My work uses this convention where we put the build files for third-party vendored libs into a single directory that has a bunch of files named things like nameofdep.BUILD. I've seen this convention used in other bazel repos as well, notably this is the same convention that Envoy uses: https://github.com/envoyproxy/envoy/tree/master/bazel/external

The way I've implemented this (by pushing another item to the auto-mode-alist) probably isn't right, it would be better to update the rx expression on the line above. But I don't really grok how to use rx, so if you want to close this PR and update the rx expression that would also make me happy.

[phst]

Thanks for the contribution, this makes sense.

To write this as rx expression, you'd probably use something like:

(rx (or "/BUILD" "/BUILD.bazel" ".BUILD") eos)