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

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -8669,8 +8669,8 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
         <entry>plus/minus sign in specified position</entry>
        </row>
        <row>
-        <entry><literal>RN</literal></entry>
-        <entry>Roman numeral (input between 1 and 3999)</entry>
+        <entry><literal>RN</literal> or <literal>rn</literal></entry>
+        <entry>Roman numeral (values between 1 and 3999)</entry>
        </row>
        <row>
         <entry><literal>TH</literal> or <literal>th</literal></entry>
@@ -8798,6 +8798,19 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
        (e.g., <literal>9.99EEEE</literal> is a valid pattern).
       </para>
      </listitem>
+
+     <listitem>
+      <para>
+       In <function>to_number()</function>, the <literal>RN</literal>
+       pattern converts Roman numerals (in standard form) to numbers.
+       Input is case-insensitive, so <literal>RN</literal>
+       and <literal>rn</literal> are equivalent.  <literal>RN</literal>
+       cannot be used in combination with any other formatting patterns or
+       modifiers except <literal>FM</literal>, which is applicable only
+       in <function>to_char()</function> and is ignored
+       in <function>to_number()</function>.
+      </para>
+     </listitem>
     </itemizedlist>
    </para>