pgsql: Add standard collation UNICODE

Add standard collation UNICODE

This adds a new predefined collation named UNICODE, which sorts by the
default Unicode collation algorithm specifications, per SQL standard.

This only works if ICU support is built.

Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://www.postgresql.org/message-id/flat/1293e382-2093-a2bf-a397-c04e8f83d3c2@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d21d4b9bc1f9da9dda29e5c4db0c6dd45408aaa

Modified Files
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doc/src/sgml/charset.sgml                      | 31 +++++++++++++++++++++++---
src/bin/initdb/initdb.c                        | 10 ++++++---
src/include/catalog/catversion.h               |  2 +-
src/test/regress/expected/collate.icu.utf8.out |  9 ++++++++
src/test/regress/sql/collate.icu.utf8.sql      |  1 +
5 files changed, 46 insertions(+), 7 deletions(-)