HPCC-9174 Allow functions that extend strings to be defined

Added as part of (aborted) changes for HPCC-9155

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4264 from ghalliday/issue9174

HPCC-9174 Allow functions that extend strings to be defined

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>