filterx: add a '+' operator skeleton that works on strings #1498

Job	Run time
create-source-tarball	1m 48s
create-packages / almalinux-8	1m 44s
create-packages / debian-bookworm	1m 42s
create-packages / debian-testing	1m 17s
create-packages / fedora-39	1m 50s
create-packages / ubuntu-noble	1m 14s
	9m 35s

Provide feedback