Add TI RJE0020A VQFN-20, correct QFN courtyard rounding

Add the TI footprint ref RJE0020A: 0.45mm pitch VQFN-20, with a very small, slightly off-centre and non-square EP.

As used on the TI TPS5139A (c.f. symbol MR kicad-symbols!3963) and the TPS568230 (same thing with low power mode).

DS: https://www.ti.com/lit/ds/symlink/tps51396a.pdf

This also includes a fix and modularisation of the D/QFN courtyard code (was giving a 0.245 mm courtyard, now gives 0.255mm)

Questions

• This has a very small EP - I am not sure I have it right
• I added a chamfer not in the DS as the long pads get rather close otherwise (geometry is also not DS-compliant in general, as it's using the IPC calculations)

I have also used a "hacked" lead length of (0.3 .. 0.3) as the datasheet lead-length (0.3 .. 0.5mm) produces extremely long pads that get very close in the corners:

Choosing 0.4 instead gives 0.8mm pads like this, which might be better?

Footprint MR: kicad-footprints!3228 (merged)