pgsql: Extend the default rules file for contrib/unaccent with Vietname

Extend the default rules file for contrib/unaccent with Vietnamese letters.

Improve generate_unaccent_rules.py to handle composed characters whose base
is another composed character rather than a plain letter. The net effect
of this is to add a bunch of multi-accented Vietnamese characters to
unaccent.rules.

Original complaint from Kha Nguyen, diagnosis of the script's shortcoming
by Thomas Munro.

Dang Minh Huong and Michael Paquier

Discussion: https://postgr.es/m/CALo3sF6EC8cy1F2JUz=GRf5h4LMUJTaG3qpdoiLrNbWEXL-tRg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ec0a69e49bf41a37b5c2d6f6be66d8abae00ee05

Modified Files
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contrib/unaccent/generate_unaccent_rules.py | 39 ++++++++--
contrib/unaccent/unaccent.rules | 114 ++++++++++++++++++++++++++++
2 files changed, 145 insertions(+), 8 deletions(-)