Add `dataclass_wizard.formats.*` functional API (loads/dumps/load/dump) for JSON/YAML/TOML/ENV

[rnag]

• Goal: Provide a non-mixin API for format conversions.

• Proposed API:

  • dataclass_wizard.formats.json.loads(cls, s, **kw) -> cls
  • ...json.dumps(obj, **kw) -> str
  • ...json.load(cls, fp, **kw) -> cls
  • ...json.dump(obj, fp, **kw) -> None
  • Same for yaml, toml, env (dotenv-style) where applicable.

• Acceptance criteria:

  • Mixins delegate to format functions (single source of truth).
  • Optional deps remain optional (ImportError with clear “install extra” message).
  • Docs: add a “Formats API” section with examples.