utf8: support East_Asian_Width

[zakuro9715]

See https://www.unicode.org/reports/tr11/

This is important for Japanese (And probably other East Asians, too)

[spytheman]

This new addition will add between 36KB and 85KB (because of all the constants), to every program that imports encoding.utf8, including the V compiler itself (it imports it through v.scanner), even if the functions are not used (-skip-unused is not good enough for now).

Can it be moved in vlib/encoding/utf8/east_asian ?