Modernize to_char's Roman-numeral code, fixing overflow problems.

Modernize to_char's Roman-numeral code, fixing overflow problems.

int_to_roman() only accepts plain "int" input, which is fine since
we're going to produce '###############' for any value above 3999
anyway.  However, the numeric and int8 variants of to_char() would
throw an error if the given input exceeded the integer range, while
the float-input variants invoked undefined-per-C-standard behavior.
Fix things so that you uniformly get '###############' for out of
range input.

Also add test cases covering this code, plus the equally-untested
EEEE, V, and PL format codes.

Discussion: https://postgr.es/m/2956175.1725831136@sss.pgh.pa.us