Add .MODULE.bazel extension to Starlark

[auscompgeek]

Description

Bazel, in some circumstances, supports splitting a MODULE.bazel file into multiple *.MODULE.bazel files with an include() directive.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.MODULE.bazel
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/GoogleContainerTools/distroless/blob/517026aa21cf5ce3a6e27f224cc93265070e3a75/private/repos/deb/deb.MODULE.bazel
    • Sample license(s):
      • Apache-2.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[auscompgeek]

I believe both .bzl and .star should both be considered the "primary" extensions for Starlark (.star being the tool-agnostic extension, .bzl being widely used in Bazel), so I'm adding this after both per the contributing docs.

[lildude]

You can only have one primary and it should be first. The rest need to be in alphabetical order.