Feat file backed dict

[Poofjunior]

Vibe-code hierarchical, saveable dict. (I made some edits and wrote the tests.) This is a useful pattern, though we might revisit it with models later. This repo seems to be the best spot for it for now.

• FileBackedDicts are dict-like objects that are created from file (yaml or json)
• Once loaded from a file, FileBackedDicts can be manipulated like dicts.
• FileBackedDicts can contain FileBackedDicts ("subdicts"). These subdicts
  have their own save() method.
  • Calling save() from a subdict only saves the keys/values in the scope
    of the subdict to the original file.
  • Calling save() from a parent dict also saves any keys/values altered
    by the child. (This is consistent with how normal dicts work.)
  • FileBackedDicts can be converted to plain dictionaries with to_dict().