Include _config.yml in a new theme's gemspec

[bglw]

This is a 🐛 bug fix.

Summary

#7304 introduced loading the _config.yml from a theme, but the gemspec doesn't include this file.

Currently a site will correctly include values from the theme's _config.yml locally, but not once the theme is packaged and pulled from a gem repository, as the file doesn't exist.

This PR adds _config.yml to the gemspec generated when you run jekyll new-theme.

Context

#7304 introduced this feature.

Notes

• I haven't added a test for this change as there currently doesn't seem to be a test around gem packaging
• All existing themes will still encounter this problem unless they amend their gemspec files to include _config.yml

[ashmaroli]

Thank you for submitting this @bigelowcc.
I should've foreseen this and documented appropriately. Better late than never.

Regarding tests, we currently cover this aspect via Cucumber features.

As for the proposed change, the feature is currently limited to just loading _config.yml at the root of the theme-gem. _config.yaml will be ignored.

[DirtyF]

LGTM

[ashmaroli]

Not merging this right away since we need to decide if Jekyll should bundle _config.yml by default.
Theme authors should ideally opt-in voluntarily to bundle the config file in the gem..

[mattr-]

If you're packaging a theme as a gem, we should make it easiest for the user to use the gem by default. Theme authors should opt-out.

@jekyllbot: merge +minor

[ashmaroli]

Theme authors should opt-out.

Note to myself: Ensure that this is mentioned in the v4.1.0 release-post since novice users won't realize they're bundling their config file right away.