fix(vson): add standard default dimensions to vson_def

[rudra496]

Summary

Fixes #782.

Previously, `vson_def` declared `p`, `w`, `grid`, `pinw`, and `pinh` without default values. Calling `fp.string("vson8")` failed with a raw Zod parse error.

Changes

1. Default Dimensions: Added standard package default dimensions to `vson_def` (`p: "0.5mm"`, `w: "3.3mm"`, `grid: "3x3mm"`, `pinw: "0.6mm"`, `pinh: "0.24mm"`), matching sibling `son` and `wson` footprint behavior.
2. Unit Tests: Added `tests/vson-bare-string.test.ts` verifying that bare string `vson8` and `vson` render 8 pads without error.

Verification

• `bun test tests/vson-bare-string.test.ts` (2/2 passing).

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