Use C99-designated initializer syntax for arrays related to encodings

Use C99-designated initializer syntax for arrays related to encodings

This updates the following lookup arrays to use C99-designated
initializer syntax, indexed based on the enum pg_enc:
pg_enc2icu_tbl[]
pg_enc2name_tbl[]
pg_wchar_table[]

This is more readable, and removes problems with ordering mistakes as
this removes dependencies between the arrays and their lookup index in
the enum pg_enc.  So, adding new encodings becomes easier, even if this
does not happen often.

Author: Jelte Fennema-Nio
Reviewed-by: Jian He, Japin Li
Discussion: https://postgr.es/m/CAGECzQT3caUbcCcszNewCCmMbCuyP7XNAm60J3ybd6PN5kH2Dw@mail.gmail.com