ts: Fix incorrect location of plugins in type definitions (`WorksheetOptions` -> `SpreadsheetOptions`)

In the TypeScript type definitions, plugins is currently defined under WorksheetOptions. However, this appears to be incorrect as plugins should be configured at the spreadsheet level (SpreadsheetOptions).

https://github.com/jspreadsheet/ce/blob/a82cb27726442dc50601183d2d943491bf531835/dist/index.d.ts#L1124-L1127