feat(linter): add schema for unicorn/prefer-at

[WaterWhisperer]

Adds verified config schema generation for unicorn/prefer-at.

Refs #22955.

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks
⏩ 66 skipped benchmarks¹

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_{Comparing WaterWhisperer:add-prefer-at-schema (d6e2326) with main (65f9233)}

Footnotes

1. 66 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩