Add xcstrings format support and export-all option

[adrianrl]

Adds Apple String Catalog (.xcstrings) support to the parser while keeping the existing .strings flow and the same generated .swift output.

Key points

• Added --format strings|xcstrings to choose the download/output translation format (maps to POEditor's type). The generated .swift file is identical for both.
• Added XCStringsTranslationParser to parse the downloaded catalog JSON into Translation, taking values from the preferred → source → any language.
• Added --exportall flag to download all languages at once (options=[{"export_all": 1}]); combined with xcstrings it produces a single multi-language catalog.
• Normalized placeholder-marked variables ({1{var}} → {{var}}) in the dumped .xcstrings so they match the iOS format, matching what .strings already did.
• The normalization uses a single linear pass, so it stays fast and normalizes every language in the file in well under a second.
• Updated README and added tests for the new parser and normalization.

How to review

1. --format strings (default) → unchanged behavior, .strings + .swift.
2. --format xcstrings --exportall → single .xcstrings with all languages, variables normalized to {{var}}, valid JSON.
3. Confirm the generated .swift is the same regardless of --format.