Add toDW, loadDW, toGeoDW, and loadGeoDW methods

[nshiab]

Summary

• Adds toDW and loadDW to push/pull CSV data between a SimpleTable and a Datawrapper chart or table
• Adds toGeoDW and loadGeoDW to push/pull GeoJSON data between a SimpleTable and a Datawrapper map
• All four methods support a custom apiKey env var name (defaults to DATAWRAPPER_KEY); toDW and toGeoDW also accept note and republish options
• loadGeoDW temporarily writes the JSON to .sda-cache/<chartId>.json (Node-compatible) and removes it after loading

Closes #1215

Test plan

• Tests in test/unit/methods/toDW.test.ts, loadDW.test.ts, toGeoDW.test.ts, loadGeoDW.test.ts run and pass when DATAWRAPPER_KEY is set in .env
• Tests are skipped gracefully when the key is absent (matching the Google Sheets test pattern)