Add the bun ecosystem file fetcher

[markhallen]

What are you trying to accomplish?

This PR adds the bun ecosystems basic files and FileFetcher.

Follow up to #11441

Part of https://github.com/github/dependabot-updates/issues/8148
Contributes to #6528

Anything you want to highlight for special attention from reviewers?

We added the zeitwerk gem for autoloading. This is used in Ruby on Rails to lazy load required files based on class and file naming conventions. A small number of shared files do not match the conventions and need to be ignored in zeitwerk and manually required as needed until the files themselves can be reorganised.

The FileFetcher is currently guarded with allow_beta_ecosystems? && Experiments.enabled?(:enable_bun_ecosystem) to control rollout and usage.

How will you know you've accomplished your goal?

rspec spec/dependabot/bun/ specs should pass.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

Deleted PR

A failed rebase caused the previous PR to close.