pgsql: Add tab-completion for ALTER TABLE not-nulls

Add tab-completion for ALTER TABLE not-nulls

The command is: ALTER TABLE x ADD [CONSTRAINT y] NOT NULL z

This syntax was added in 18, but I got pushback for getting commit
dbf42b84ac7b in 18 (also tab-completion for new syntax) after the
feature freeze, so I'll put this in master only for now.

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reported-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari(at)ilmari(dot)org>
Discussion: https://postgr.es/m/d4f14c6b-086b-463c-b15f-01c7c9728eab@oss.nttdata.com
Discussion: https://postgr.es/m/202505111448.bwbfomrymq4b@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a604affaded028b6bfba024127931289c2b756c2

Modified Files
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src/bin/psql/tab-complete.in.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)