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(at)j-davis(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/1293e382-2093-a2bf-a397-c04e8f83d3c2(at)enterprisedb(dot)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(-)