pgsql: Support RN (roman-numeral format) in to_number().

Support RN (roman-numeral format) in to_number().

We've long had roman-numeral output support in to_char(),
but lacked the reverse conversion. Here it is.

Author: Hunaid Sohail <hunaidpgml(at)gmail(dot)com>
Reviewed-by: Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Discussion: https://postgr.es/m/CAMWA6ybh4M1VQqpmnu2tfSwO+3gAPeA8YKnMHVADeB=XDEvT_A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/172e6b3adb2e2749883ad0746172e4cf97139961

Modified Files
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doc/src/sgml/func.sgml | 17 ++-
src/backend/utils/adt/formatting.c | 275 +++++++++++++++++++++++++++-------
src/test/regress/expected/numeric.out | 78 ++++++++++
src/test/regress/sql/numeric.sql | 33 ++++
4 files changed, 350 insertions(+), 53 deletions(-)