Add WSON-12-1EP_3x3mm_P0.5mm_EP1.5x2.5mm to wson.yaml

About

The package was used by TPS63710.

Footprint URL

https://www.ti.com/lit/ds/symlink/tps63710.pdf#page=33

Generator has the result:

1. No thermal vias
2. [2, 3] thermal vias