Add WQFN-20-1EP_4x4mm_P0.5mm_EP2.7x2.7mm_ThermalVias

Copied from WQFN-24-1EP_4x4mm_P0.5mm_EP2.7x2.7mm_ThermalVias.kicad_mod.

This is a footprint for TPA6130A2RTJ (https://github.com/KiCad/kicad-symbols/pull/1621).

Datasheet: http://www.ti.com/lit/ds/symlink/tpa6130a2.pdf (page 38).

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• Provide a URL to a datasheet for the footprint(s) you are contributing
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