Fix #4475: Correct FormatEnum type keys jp2→jp2k and v→vips

The `FormatEnum` TypeScript interface declared `jp2` and `v` as keys, but the runtime `sharp.format` object uses `jp2k` and `vips` (matching libvips internal format identifiers). ## Changes - `lib/index.d.ts`: Renamed `jp2` → `jp2k` and `v` → `vips` in `FormatEnum` interface - `test/unit/util.js`: Added two test cases verifying the correct keys exist and the old incorrect keys are undefined ## Test Added `uses jp2k as format key, not jp2 (#4475)` and `uses vips as format key, not v (#4475)` tests that: 1. Verify `sharp.format.jp2k` exists with correct id 2. Verify `sharp.format.vips` exists with correct id 3. Verify `sharp.format.jp2` and `sharp.format.v` are undefined Full test suite passes on macOS ARM (Apple Silicon).