zemacs: new `query-replace-regexp' function.

* lib/zemacs/zlisp/search.zl (do_query_replace): New function,
factored out of query-replace.
(query-replace): Simplify accordingly.  Improve docstring.
(query-replace-regexp): Same as `query-replace', but search
string is treated as a regular expression.
* lib/zz/commands/search.lua (do_query_replace, query_replace,
query_replace_regexp): Keep feature parity in Zz too.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>