replace enhancements

[timotheecour]

• detect errors at CT instead of RT if both regex pattern and replace pattern are given at CT
  eg:
  echo "abcfoodef".replace(re"f(o)o", "[$2]") should give a CT error

• error in replacement pattern should be more useful instead of just invalidFormatString ; in above example could show:
  error in replace pattern "[$2]" at column 2: out of bound error 2: expected values 1 through 1

• $0 could be supported with meaning of "whole match"

• other patterns could be used for before match and after match, as in https://dlang.org/phobos/std_regex.html#Replace%20format%20string ; either using D's

$` | part of input preceding the match.
-- | --
$' | part of input following the match.

or perhaps using more explicit / easier to remember names, eg $(pre) $(post)