Add basic support for REPO.bazel and VENDOR.bazel files (#1485)

## Buildtools PR checklist

- [x] The code in this PR is covered by unit/integration tests.
- [x] I have tested these changes and provide testing instructions
below.
- [x] I have either responded to, or resolved all Gemini comments on the
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- [x] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
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## Description

Add basic support for REPO.bazel and VENDOR.bazel files

As these files are pretty trivial, we mostly treat them as generic
BUILD-like files and only wire up type detection and the Buildifier
--type flag.

### (optional) These changes were tested using the following steps