Treat .build_defs and BUILD as Bazel/Starlark files.

Created by: slimsag

A customer https://app.hubspot.com/contacts/2762526/company/2174376075/ uses please build which defines BUILD files and .build_defs files in Starlark (Bazel's language). Bazel also uses BUILD files.

Today, we only treat .bzl as Bazel/Starlark files (both in our syntax highlighter and e.g. when using lang:bazel I believe), the ask here is to also treat BUILD and .build_defs files as Bazel/Starlark by default / out of the box.

Ref. https://sourcegraph.atlassian.net/jira/servicedesk/projects/SG/queues/custom/1/SG-25