feat(extraField)!: support duplicate keys, unify parser option, move save to options, avoid empty __nonStandard__

[northword]

✅ Changes

1. Support multiple values for the same key

   • getExtraFields(item, "custom") now returns Map<string, string[]> to allow multiple identical keys (e.g. multiple original-author entries).
   • getExtraFields(item, "default") overload remains compatible with Zotero's api and returns Map<string, string>.

2. Move save to options: Both setExtraField and replaceExtraFields now use an options object instead of a standalone save argument (introduced in feat(extra-field-tool): allow disable save #95) for cleaner and extensible API design.

3. Parser option: parser: "enhanced" | "classical" replaces the previous backend parameter.

   • enhanced: enhanced custom parser (multi-value support), original custom
   • classical: Zotero built-in parser (single value per key), original default

4. Fixed __nonStandard__ handling

   • Prevented adding __nonStandard__: [""] when no non-standard lines exist in the extra field.

💥 BREAKING CHANGES

• getExtraFields(item, "custom") now returns Map<string, string[]> instead of Map<string, string>. Code that previously expected a string value must now handle arrays.
• The save argument in setExtraField and replaceExtraFields has been replaced by an options object ({ save?: boolean }).
• parser replaces backend:
  • "enhanced" → custom enhanced parser
  • "classical" → Zotero built-in parser