Add WORKSPACE.bzlmod to Starlark (#7121)

* Make .bzlmod handled by Starlark

Bazel's bzlmod dependency manager involves Starlark files with `.bzlmod` file extensions.

https://bazel.build/external/migration#workspace.bzlmod

* Adjust per code review

* Add sample file

Author: petemounce

lib/linguist/languages.yml

@@ -7056,6 +7056,7 @@ Starlark:
   - Tiltfile
   - WORKSPACE
   - WORKSPACE.bazel
+  - WORKSPACE.bzlmod
   aliases:
   - bazel
   - bzl

samples/Starlark/filenames/WORKSPACE.bzlmod

@@ -0,0 +1,6 @@
+# This file marks the root of the Bazel workspace.
+# See MODULE.bazel for dependencies and setup.
+# It is only used when bzlmod is enabled.
+# This file will generally be kept empty in favor of using MODULE.bazel fully.
+
+workspace(name = "build_bazel_rules_ios")