feat(toml): fix JSON to TOML root scope and null handling

[LovesAsuna]

fix #2618
Summary of the change:

- Updated TOML root encoding so root-level scalar fields and regular arrays are emitted before any TOML table headers.                                           
  - This keeps fields like sort = [1] at the TOML root level.                                                                                                    
  - Reason: in TOML, once [highlight] or [[_source.cookie]] is emitted, later key-value pairs belong to that table. TOML has no syntax to “return” to the root   
table.
- Preserved array-of-table behavior for object arrays.
  - _source.cookie still becomes:
  [[_source.cookie]]
  - Reason: arrays of JSON objects should continue mapping to TOML array-of-table syntax.
- Changed JSON null handling for TOML output.
  - Null-valued object fields like Unparsed: null are now skipped instead of emitted as:
  Unparsed = ""
  - Reason: TOML has no native null, and converting null to an empty string changes the meaning.
- Added regression coverage for the reported JSON shape.
  - The test checks that sort stays root-scoped, _source.cookie is encoded correctly, and Unparsed is omitted.

In short: the fix prevents root fields from leaking into the previous TOML table and avoids incorrectly converting JSON nulls into empty strings.