Add support for the Sipeed Tang Primer 20k (GW2A)

[zapta]

Hi @cavearr, do the definition files boards.json, fpgas.json, and blinky.cst look good? Does it work with the Primer 20K board you have?

work-tang-20k-primer.zip

(Currently Apio uses yosys 2025-09-23, possibly we need to move it forward to support the Primer 20k).

[zapta]

Once we confirm that the example works I will add the example to fpgawars/apio-examples and the .json files to fpgawars/apio-definitions.