Create WQFN-20-1EP_3x3mm_P0.4mm_EP1.8x1.8mm_ThermalVias

[eclecticc]

Footprint from https://www.ti.com/lit/ds/symlink/tpd6s300a.pdf#page=32

Note that this is also called S-PWQFN-N20 by TI, but there is an existing footprint for that in the library that doesn't match TI's drawing.

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All contributions to the kicad library must follow the KiCad library convention

Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:

• [x] Provide a URL to a datasheet for the footprint(s) you are contributing
• [x] An example screenshot image is very helpful
• [x] If there are matching symbol or 3D model pull requests, provide link(s) as appropriate
• [x] Check the output of the Travis automated check scripts - fix any errors as required
• [x] Give a reason behind any intentional library convention rule violation.

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Be patient, we maintainers are volunteers with limited time and need to check your contribution against the datasheet. You can speed up the process by providing all the necessary information (see above). And you can speed up the process even more by providing a dimensioned drawing of your contribution. A tutorial on how to do that is found here: https://forum.kicad.info/t/how-to-check-footprint-correctness/9279 (This is optional!)