pgsql: Use C99-designated initializer syntax for arrays related to enco

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ada87a4d95fc39dfb1214edf6653390314b6f0df

Modified Files
--------------
src/common/encnames.c | 155 +++++++++++++++++++++++-----------------------
src/common/wchar.c | 85 +++++++++++++------------
src/include/mb/pg_wchar.h | 8 +--
3 files changed, 122 insertions(+), 126 deletions(-)