Deep Merge of Frontmatter

[rickychilcott]

Bridgetown Version:
bridgetown 1.2.0.beta5 "Bonny Slope

To Reproduce
Create a resource with multiple levels of frontmatter:

---
example:
  from_resource: "from resource"
---

and also set config defaults:

defaults:
  - scope:
      path: "articles"
    values:
      layout: default
      example:
        from_config: "from config"

The defaults get totally clobbered and you don't get a value at resource.data.example.from_config

See example repo: https://github.com/rickychilcott/bridgetown-deep-merge-test

Current behavior
Last one wins

Expected behavior
Deep merge of front matter.

Computing environment (please complete the following information):

• OS: Mac OS X
• Browser: Brave
• Browser Version: Version 1.46.153 Chromium: 108.0.5359.128 (Official Build) (arm64)
• Ruby Version: 3.0.3p157

Additional context
I looked around the source to try to figure out where to write a failing test and ultimately a fix, but I couldn't find where to fix it. I did see the Utils.deep_merge_hashes utility class and it being used, but wasn't sure where to plug it in or the specs to modify to drive out the behavior.